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Review

Transition Metal Selenite Halides: A Fascinating Family of Magnetic Compounds

by
Peter S. Berdonosov
*,
Elena S. Kuznetsova
and
Valery A. Dolgikh
Department of Chemistry of Lomonosov Moscow State University, Moscow 119991, Russia
*
Author to whom correspondence should be addressed.
Crystals 2018, 8(4), 159; https://doi.org/10.3390/cryst8040159
Submission received: 12 March 2018 / Revised: 31 March 2018 / Accepted: 2 April 2018 / Published: 4 April 2018
(This article belongs to the Special Issue Crystal Structures of Compounds Containing Ions Selenite)
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Abstract

:
The problem of searching for low-dimensional magnetic systems has been a topical subject and has attracted attention of the chemistry and physics community for the last decade. In low-dimensional magnetic systems, magnetic ions are distributed anisotopically and form different groups such as dimers, chains, ladders, or planes. In 3D frameworks, the distances between magnetic ions are equal in all directions while in low-dimensional systems the distances within groups are different from those between groups. The main approach of searching for desired systems is a priori crystal chemical design expecting the needed distribution of transition metal ions in the resulting structure. One of the main concepts of this structural design is the incorporation of the p-element ions with stereochemically active electron pairs and ions acting as spacers in the composition. Transition metal selenite halides, substances that combine SeO32− groups and halide ions in the structure, seem to be a promising object of investigation. Up to now, there are 33 compounds that are structurally described, magnetically characterized, and empirically tested on different levels. The presented review will summarize structural peculiarities and observed magnetic properties of the known transition metal selenite halides. In addition, the known compounds will be analyzed as possible low-dimensional magnetic systems.

1. Introduction

In recent decades, approaches for the search for and synthesis of low-dimensional magnets, that is, the systems with magnetic ions distributed anisotropically in the crystal structure and forming different types of groups as dimers, trimers, tetramers or isolated plaquette units, chains of ions, ladders, or planes, have been attracting considerable interest from the physics and chemistry research communities. Such geometric distribution of magnetic ions may lead to exotic magnetic states such as spin liquids or geometric magnetic spin frustrations which are hot topics in the physics of condensed matter [1,2]. In the usual three-dimensional structures, the magnetic exchange between magnetic ions is equal in all directions whereas in low-dimensional systems the exchange within the above-mentioned groups is much stronger than between the groups.
The interest in low-dimensional magnetic systems has resulted, on the one hand, from their fundamental importance as model objects for the verification of theoretical predictions in condensed matter physics theory and on the other hand, from their possible applications, for example, for tunable heat conduction discussed in [3]. Until now, the compounds studied from this point of view in inorganic chemistry are usually limited to oxide systems, chalcogenide compounds, and sometimes halides [2]. Currently, one of the main avenues for the search for low-dimensional magnetic compounds is the targeted synthesis of new compounds using the a priori structure design suggesting a special distribution of magnetic transition metal ions in the final crystal structure. It should be kept in mind that a structural sublattice of ions could be different from the real magnetic exchange path but the probability to find a low-dimensional magnetic sublattice in a low-dimensional crystal structure should be higher. The structural influence of stereochemically active electron pairs of p-elements in the middle oxidation states, such as Bi(III), Pb(II), Te(IV), Se(IV) etc. appears to be an unattractive synthesis concept in the search for low-dimensional magnetic systems. A lone electron pair is non-bonding and plays a role of the additional ligand for the central ion. According to the Valence Shell Electron Pair Repulsion theory [4,5] such ions in the oxidation state +4 should have a stereochemically active electron pair which distorts the selenium environment. As a result, one side of the ion is not bonded to other atoms in the crystal structure and electron pairs form cavities or holes in the crystal structure. According to [6], the volume of the non-bonding pair of electrons is compatible to that of the O2− ion. Sometimes [7] ions with electron pairs are referred to as ‘chemical scissors’ which are “cutting through” the 3D crystal structure. The resulting structure cavity may be extended even to a layer by introducing the additional terminating ions (spacers), for example, halide ions.
By now, many transition metal selenite halides have been synthesized and characterized. The majority of these compounds contain copper in the oxidation state +2. Some compounds in the mentioned group have been characterized magnetically by different techniques. In accordance with the above-mentioned concept, it appears useful to summarize the information about the known compounds including their crystal structures and magnetic characterization, which will be the subject of the present review.
Usually the selenite group SeO32− possesses a pyramidal shape (C3v symmetry in the free state) with distances Se–O are about 1.73 Å and O–Se–O angles are 103° [8]. In addition, the SeO32− group may be described as a pseudo-tetrahedral anion with the lone electron pair acting as an invisible ligand in the crystal structures. The observation of lone electron pairs in selenites is one of their structural peculiarities. In the solid state, the C3v symmetry of SeO32− groups is distorted due to the differences in the connectivity. In contrast to sulfur, the +4 oxidation state is more stable for selenium due to the small value of redox potential SeO42−/SeO32− 0.05 V which is positive in alkaline solutions whereas the corresponding value for SO42−/SO32− is much lower −0.93 V. This makes selenites more stable in solutions compared to sulfites [8,9]. Tellurium has a larger radius and may form not only TeO32− tellurite groups but display higher coordination numbers, 3 + 1 for example. Moreover, tellurites often tend to form polyions with different compositions and geometry [7,10].
Combining the transition ions, selenite, or tellurite groups with halide ions within one structure may lead to the novel crystal structures and different magnetic properties [11,12,13,14,15]. It is easy to note that up to now transition metal tellurite halide compounds are more explored. There are two factors which account for this. First one is the oxidizing activity of selenites. In water solution, the reduction potential SeO32−/Se is −0.336 V [9]. For this reason, it will be difficult to combine Ti3+, V3+ ions with selenites, while tellurites tend to be more stable. Another reason for tellurites being more studied is the stability of tellurium dioxide in the air and consequently its facile synthetic preparation. Selenium dioxide is air- and moisture-sensitive, hence all preparative procedures with it are usually performed under dry conditions (i.e., argon or nitrogen filled glove boxes), in order to avoid the formation of hydrates or hydroselenites. As it will be shown below, chlorides are the most representative group among halides, whereas iodides were obtained in one case only. The reason for this seems to be lying in the oxidizing effect of the selenite groups.
Usually, transition metal ions are responsible for the magnetic behavior of substances. From this point of view, 3d metals in middle oxidation states are most attractive as 4d and 5d metals in low oxidation states may bind together and the magnetic moment of metal ions disappears [16]. The literature search shows that up to now there are no data about crystal structures and magnetic properties of 4d and 5d transition metal selenite halides (taking into account that compound Pb2NbO2(SeO3)2Cl [17] contains Nb in the oxidation state +5 and d0 configuration). Therefore, in this review we will describe crystal structures and some reported magnetic properties of 3d metal selenite halides, that is, compounds containing both SeO32– groups and halide ions.

2. Crystal Structures and Magnetic Properties Overview

Here we will describe the crystal structures of complex transition metal selenite halides with known magnetic properties. The term “halide ions” will refer to chlorides, bromides, and sometimes iodides. Fluoride compounds usually show different behavior and tendencies due to the peculiarities of fluorine and fluorides sometimes are closer to pure oxide compounds. Moreover, the literature data about selenite fluorides or oxofluorides of d-metals are rather limited and describe only a small number of compounds [18,19,20,21].
Below, we will pay special attention to the transition metal M–M ions distances in the structures. It should be noted, that this is an oversimplified way of considering the spin interactions. For a precise estimation, the geometry of d-orbitals and their interactions with the orbitals of bridged atoms should be examined. Nevertheless, it seems that presented simple model may be used at least for crystal chemical design of 3d-metal selenite halide compounds, as in the case of very long M–M distances the probability of spin interactions exchange should be lower.

2.1. Composition MII5(SeO3)4X2

The group of compounds under consideration contains 6 different compositions and exists up to now only for Fe, Co, Ni, and Cu. Compositions and some crystallographic information are given in Table 1. As can be seen from Table 1, the crystal structures for these compounds are different in compositions and cell symmetries.
The substances may be divided into several groups according to their common composition and structural relations.
The first group of transition metal selenite halides may be described by the composition MII5(SeO3)4X2 (substances 1–5 in Table 1) [22,23,24,25]. While cobalt and nickel compounds are similar in some forms, copper compounds’ crystal structures are different. The crystal structure of Cu5(SeO3)4Cl2 is presented in Figure 1 as the projection on ab plane.
The crystal structure of Cu5(SeO3)4Cl2 [20] may be presented as a three-dimensional framework of corner- and edge-shared polyhedra [CuO5], [CuO6], and [CuO3Cl2]. It is easy to see in Figure 1 that the framework of Cu polyhedra contains wide holes where selenium atoms are located. Due to the pyramidal shape and the presence of the E-pairs in SeO32– groups copper polyhedra do not polymerize in more dense packing. According to the structural data, Cu–Cu distances in the crystal structure of Cu5(SeO3)4Cl2 are in the range of 3.15–3.30 Å and one distance value is about 3.6 Å. One may conclude that spin–spin interactions between Cu2+ ions may be observed in this compound.
Field-cooled magnetization measurements data show spontaneous polarization below the critical temperature of TC ≈ 45 K. Below this point almost linear magnetization increase and saturation at low temperatures are observed. The heat capacity temperature dependence demonstrates a λ-type anomaly at 44.1(2) K thus confirming magnetization measurements results. The Curie–Weiss temperature calculated from the approximation of experimental data has a value of −151(4) K and indicates predominantly antiferromagnetic spin-exchange interactions. Thus, a long-range antiferromagnetic order occurs in Cu5(SeO3)4Cl2 at 44.1(2) K.
However, field dependent magnetization measurements at 1.87 K reveal a hysteresis loop, indicating the presence of weak ferromagnetic interactions. This is a common phenomenon occurred in complex magnetic systems based on Cu2+ ions where low-dimensional magnetic interactions take place. The complexity of exchange paths is due to the numerous distortions of coordination polyhedra in the crystal structure. Analysis of interconnections between copper, chlorine, and oxygen atoms in the Cu5(SeO3)4Cl2 structure revealed two main types of magnetic exchange paths: the first one is the antiferromagnetic path between copper atoms connected via the oxygen atom; the Cu–O–Cu angle value exceeds 90°. The second path is the ferromagnetic path between copper atoms interacting via a chlorine atom. The Cu–Cl–Cu angle is almost 90°—such bonding angles typically support ferromagnetic spin-exchange interactions [22].
Due to the Jahn–Teller effect, the surrounding of Сu2+ differs from that of Co2+ and Ni2+, and this is the reason for differences between cobalt and nickel compounds and the copper ones. Similar compounds for cobalt and nickel have common structures at least in one form (Table 1, substances 2, 3 and 5 [23,25]). All three metal atoms in the compounds under discussion have a distorted octahedral coordination (Figure 2a). Two atoms have a [MO5X] (X = Cl and Br) coordination and one M atom has a [MO4X2] coordination with the oxygen atoms in the square plane. The O–O edge-sharing [MO5X] polyhedra form chains which are connected via common halogen vertexes into layers in the crystal structures of cobalt and nickel compounds. A three-dimensional structure appears when such layers are bridged by [MO4X2] polyhedrons and SeO32– groups (Figure 2a). The crystal structures of Co5(SeO3)4X2 and Ni5(SeO3)4Br2 have M–M distances in the range of 3.10–3.37 Å for Co5(SeO3)4Cl2 [23], 3.07–3.40 Å for Co5(SeO3)4Br2 [23], and 3.03–3.39 Å for Ni5(SeO3)4Br2 [25].
In spite of the similarity of Co5(SeO3)4X2 (X = Cl and Br) compounds, the nickel chloride is not isostructural with the bromide one [24,41]. The unit cell for Ni5(SeO3)4Cl2 is about twice as big in volume compared to that of the bromide. There are five Ni atoms in the structure which are octahedrally coordinated by oxygen atoms from SeO32− groups or Cl– ions. There are three types of [NiO5Cl] and one type of [NiO4Cl2] octahedrons and a pure oxygen octahedron [NiO6] in the structure of Ni5(SeO3)4Cl2 (Figure 2b). Thus, the structure of Ni5(SeO3)4Cl2 features a condensed three-dimensional (3D) network built by Ni2+ ions interconnected by SeO32– anions as well as Cl– anions. The crystal structure of Ni5(SeO3)4Cl2 is presented in Figure 2b. It is interesting to mention that Ni–Ni distances in nickel selenite chloride vary from 2.94 up to 3.9 Å [41]. The existence of isostructural compounds for Co with Cl and Br may indicate that Ni5(SeO3)4Cl2 might form two polymorphs. It should be noted that tellurites of the similar composition Ni5(TeO3)4X2 (X = Cl, Br) [12] are isostructural to each other as well as to related cobalt compounds Co5(TeO3)4X2 (X = Cl, Br) [42], but differ from presented selenites. Magnetic properties were studied for Ni5(SeO3)4Cl2 as well as for isostructural compounds Co5(SeO3)4X2 (X = Cl and Br) and Ni5(SeO3)4Br2.
Ni5(SeO3)4Cl2 demonstrates antiferromagnetic properties, according to magnetic susceptibility measurements data. Long-range order takes place at approximately 4.5 K. Curie–Weiss temperature θ = −143(2) K indicates strong antiferromagnetic interactions between nickel atoms. Such strong interactions take place because of the very short Ni–Ni distances. Moreover, the crystal structure of Ni5(SeO3)4Cl2 is three-dimensional and the network is highly condensed. According to the magnetic measurements data there is no evidence of extraordinary or unusual magnetic behavior in this compound [22], but up to now no detailed study has been undertaken, therefore this problem is still of a certain interest.
Concerning Co5(SeO3)4X2 (X = Cl and Br) and Ni5(SeO3)4Br2, there are also antiferromagnetic transitions in the magnetic subsystems of these compounds (TN = 18 K, 20 K and 46 K for Co5(SeO3)4Cl2, Co5(SeO3)4Br2, and Ni5(SeO3)4Br2, respectively) [23,25]. However, unlike Ni5(SeO3)4Cl2, magnetic measurements were performed using single crystal samples, and this allowed to carry out a more detailed investigation. In particular, the magnetic susceptibility measurements in oriented crystals revealed anisotropy—typical for the zero-field splitting—induced by the single-ion anisotropy of 3d-metal ions. This allowed defining ground states and electron transitions in these compounds at lower temperatures. Another peculiarity is that the magnetic ordering permeates only one subsystem in the complex magnetic system: a minority fraction is represented in Co5(SeO3)4X2 (X = Cl and Br) ordered subsystem while a magnetically ordered fraction dominates in Ni5(SeO3)4Br2.

2.2. Compositions Sr2MII(SeO3)2Cl2 and MM”2(SeO3)2Cl2

The group of selenites with the composition Sr2MII(SeO3)2Cl2 is presented by isostructural compounds for MII = Co, Ni, and Cu [26,27] (compounds 6–8 in Table 1). The unit cell of these compounds contains one independent atom of Sr and one atom of M. The nearest coordination surrounding of Sr and M atoms comprise oxygen and chlorine atoms, whereas Se atoms have only oxygen atoms as neighbors. In all three compounds, strontium atoms are situated in distorted [SrO6Cl3] 9-vertex polyhedra which form layers perpendicular to the [0 0 1] direction of the structure. d-metal ions are allocated in Jahn–Teller distorted octahedrons [MO4Cl2]. These polyhedra share common edges with layers of Sr polyhedra and form a 3D structure shown in Figure 3. The M–M distances in the represented structures are longer than 5 Å.
The substitution of one more strontium ion by a d-element ion for compounds Sr2MII(SeO3)2Cl2 is possible for Cu2+ and produces SrCu2(SeO3)2Cl2 [27]. This compound is similar to the tellurites SrCu2(TeO3)2Cl2 [43] and BaCu2(TeO3)2Cl2 [44]. These compounds crystallize in the non-centrosymmetrical space group P21. The crystal structure of SrCu2(SeO3)2Cl2 is given in Figure 4. The asymmetric unit of SrCu2(SeO3)2Cl2 contains one Sr atom and two copper atoms. Sr atom is allocated in the [SrO6Cl2] polyhedron and copper atoms are situated in distorted square pyramids [CuO4Cl] and [CuO3Cl2]. It seems that Jahn–Teller distortion in the compound under discussion is much stronger than for a copper atom surrounding in the case of Sr2Cu(SeO3)2Cl2 [27]. Such a strong distortion is impossible for cobalt and nickel and may explain why the same structures are not formed for them. Both Se atoms in the structure are connected to the strontium or copper polyherda and play the role of an additional linker to form a framework. Copper polyhedra are connected by the common O–Cl edges and form dimers [Cu2O4Cl2] (Figure 4). The distance of Cu–Cu in the dimer groups is 3.29 Å which is shorter than 3.35 Å in SrCu2(TeO3)2Cl2 [43].
According to Shannon’s data [45] the ionic radius of Sr2+ ions is close to that of Pb2+. At the same time, according to the Periodic Table of the elements, calcium as an alkali earth metal may be a smaller analog of strontium. There were obtained compounds with compositions CaCu2(SeO3)2Cl2 and PbCu2(SeO3)2Cl2 [28,29] (Table 1 substances 9 and 11). It is apparent (Table 1) that both compounds are isostructural but have a different unit cell in comparison with SrCu2(SeO3)2Cl2 (Table 1 substance 10). At the same time, the tellurite compound PbCu2(TeO3)2Cl2 [29] is isostructural to SrCu2(SeO3)2Cl2 [27]. This phenomenon may be associated with differences in ionic radii of Se4+ and Te4+. A bigger tellurium atom is more compatible with strontium or lead, whereas the smaller selenium atom is the reason for the formation of the related calcium and lead compounds.
The differences in the crystal structures of these two types of compounds are shown in Figure 5, using PbCu2(YO3)2Cl (Y = Se, Te) as an example. The crystal structures of CaCu2(SeO3)2Cl2 and PbCu2(SeO3)2Cl2 contain two different copper atoms in a strongly squeezed tetrahedral [CuO4] and octahedral [CuO4Cl2] environments. In the case of octahedrally coordinated copper atoms, two oxygen atoms are situated at distances about 2.6 Å and [CuO2Cl2] may be considered as a magnetic plaquette. Calcium or lead atoms are situated in the 8-vertex polyhedrons [MO4Cl4]. Selenium atoms share common O vertexes of copper and calcium or lead polyhedra and are situated next to the holes of the framework formed by them. From data presented in [28,29] it may be seen that Cu–Cu distances are relatively long. In case of the Ca compound the distances are 3.85–4.12 Å, and for the Pb compound the distances are 3.93–4.06 Å.
Within this wide variety of compositions, magnetic properties are known only for CaCu2(SeO3)2Cl2 [28], PbCu2(SeO3)2Cl2 [29], SrCu2(SeO3)2Cl2, and Sr2Cu(SeO3)2Cl2 [46]. Results of magnetic susceptibility measurements in the wide temperature range for CaCu2(SeO3)2Cl2 indicate the low-dimensional character of magnetic behavior with predominant antiferromagnetic interactions for this compound (Figure 6). Antiferromagnetic ordering takes place at approximately 6 K. The χ(T) curve approximation resulted in a uniform spin-1/2 chain model for the magnetic structure. A strong intrachain exchange coupling of ~133 K and frustrated interchain couplings realized via the two non-equivalent superexchange paths take place in the magnetic structure (Figure 7). It is worth mentioning that structural and magnetic chains are not the same in CaCu2(SeO3)2Cl2: due to the peculiar crystal structure, the spin chains run in the direction almost perpendicular to that of the structural chains (Figure 8).
PbCu2(SeO3)2Cl2 is isostructural to CaCu2(SeO3)2Cl2 and its magnetic behavior is somewhat similar to that of the calcium compound (Figure 9). It can be approximated by a uniform spin-1/2 chain model with interchain interactions. As in the calcium compound, magnetic chains do not coincide with the structural ones (Figure 10). However, the interchain couplings in PbCu2(SeO3)2Cl2 and CaCu2(SeO3)2Cl2 are rather different: the larger value of the interchain exchange integral in PbCu2(SeO3)2Cl2 results in an increase of Neel temperature (13 K in PbCu2(SeO3)2Cl2 versus 6 K in CaCu2(SeO3)2Cl2).
Though Sr2Cu(SeO3)2Cl2 has a different structure from MM”(SeO3)2Cl2 compounds, its magnetic behavior can also be described in terms of the Heisenberg uniform spin chain with strong interchain interactions (J ≈ 104–106 K). Unlike CaCu2(SeO3)2Cl2 and PbCu2(SeO3)2Cl2, in Sr2Cu(SeO3)2Cl2 there is presumably no long-range order observed down to at least 2 K, and magnetic exchange paths lie through the structural chains built of copper polyhedra and SeO32– groups [46]. Further investigation is needed to establish the microscopic magnetic model and to define exchange paths in the magnetic subsystem.
A different case is with SrCu2(SeO3)2Cl2. Here, the most appropriate model is the one of slightly interacting dimers. This is due to the structural features including CuO4 corner-shared plaquettes. In general, according to preliminary measurements data the magnetic behavior of SrCu2(SeO3)2Cl2 is similar to that of SrCu2(TeO3)2Cl2. Nevertheless, further research should be undertaken in order to reveal special features inherent to this phase.

2.3. Compound Bi2Fe(SeO3)2OCl3

As seen from the above-mentioned data, the majority of selenite halides with described magnetic properties were studied in compounds featuring copper in the oxidation state 2+ which have S = 1/2. An example of a compound where ions with larger spin values may form magnetic sublattice is selenite oxochloride of iron and bismuth Bi2Fe(SeO3)2OCl3 [30] (Table 1 compound 12). The main feature of the crystal structure of this compound is the existence of the zig-zag chains of corner-shared [FeO6] octahedra which are decorated by [BiO4Cl3], [BiO3Cl3] polyhedra and SeO32– groups as shown in Figure 11. The selenium atoms are connected to the oxygen atoms from different [FeO6] octahedra and an additional link chain. Different iron octahedron chains are bound by Bi-polyhedra into the layer in the ab plane of the structure as depicted in Figure 11. Finally, the distance between nearest Fe3+ ions in the chain is about 3.56 Å and between Fe3+ ions from different chains and layers are 8.570(2) and 8.604(2) Å. According to this structure’s peculiarity, it may be suggested that the compound will be magnetically quasi-one-dimensional.
An extensive study is required to establish the character of magnetic behavior in this compound. The temperature dependence of magnetic susceptibility displays broad maximum at T ≈ 130 K, which indicates the formation of the short-range magnetic order (Figure 12). At lower temperature TN = 13 K, long-range antiferromagnetic ordering sets in, and this is confirmed by means of magnetization and specific heat measurements as well as by Mössbauer spectra. ESR spectroscopy data point out the quasi-1D nature of magnetic interactions. The first-principle calculations allowed defining of the main spin-exchange paths in the magnetic structure based on weakly interacted spin-5/2 zig-zag chains with strong intrachain interactions (Figure 13). Thus, a compound with a higher spin magnetic system has been demonstrated to be a low-dimensional magnet.

2.4. Francisite Like Compounds Cu3M(SeO3)2O2X

The Table 1 shows that the most representative group of compounds with the same structure symmetry and relative cell constants is formed by compounds with common composition Cu3M(SeO3)2O2X [31,32,33,34,35,36,37] (Compounds 13–30 Table 1). All these compounds are isostructural to the rare nature mineral Francisite Cu3Bi(SeO3)2O2Cl [47]. Later, its synthetic analogs for Cl, Br, and I were prepared in laboratory conditions [31]. It was shown that rare Earth metals may form compounds with francisite-like structures [32,33,34,35,36]. The crystal structure of francisite contains two copper ions positions with square planar oxygen atom coordination. Bismuth or rare Earth metal ions are in distorted cubic surroundings [MO8]. The square planes [CuO4] form kagome-like nets in the crystal structure (Figure 14). As can be seen in the Figure 14 selenite groups connect [CuO4] planes and [MO8] cubes into the 3D framework with the channels running along [0 0 1] direction. Halide atoms are situated inside these channels. The nearest distance from Cl atom is Cu–Cl which is more than 3 Å for Cu3Bi(SeO3)2O2Cl (Figure 14). The most recent studies have shown that francisite-like compounds may be formed for all are Earth Metals from La up to Lu and Y in case of chlorides [35,36] and for La till Gd in case of bromides [36]. This phenomenon may be explained by the halide ion size factor where bigger bromide ions could not be combined with smaller Rare Earths and the structure could not be formed [36]. The Kagome net in the francisite-like structures is not planar and form waves as shown in Figure 15. The main attractive structural feature of francisite-like compounds is that the variation of M3+ ion may affect Cu–Cu distance in the Kagome nets. One would expect that Cu–Cu contact distances should decrease with decreasing Ln3+ ionic radii. The experimental data summarized for chlorides in Figure 16. support such an approach but Cu1-Cu2-Cu1 angle of the net shows a more complex dependence. There are two Cu–Cu distances in the net. The Cu–Cu distances in chloride compounds are within the range of 3.13–3.23 Å. Figure 16 shows that the differences in the Cu–Cu distances decrease together with decreasing of Ln3+ radii. The variation of M3+ and of halide ions provides an opportunity for the copper sublattice tuning. Substitution of selenium by tellurium produced the compound Cu3Bi(TeO3)2O2Cl [48]. However, the copper Kagome net in the tellurite is more distorted and one copper atom has become pyramidally coordinated [CuO4Cl]. The existence of such a wide group of compounds with similar structures, together with the possibility to examine the influence of the f-element magnetic nature on the magnetic behavior of the copper ions sublattice stimulate the research of this group of substances.
The discussion of the magnetic properties of the francisite family representatives should be started with the consideration of the synthetic analogs of this mineral with the composition Cu3Bi(SeO3)2O2X (X = Cl, Br). According to magnetic susceptibility measurements data, antiferromagnetic ordering in spin-1/2 copper subsystem takes place at 23.5(5) K [31] and 27.4(5) K [49] for Cu3Bi(SeO3)2O2Cl and Cu3Bi(SeO3)2O2Br, respectively. The magnetic ground state can be described as distorted kagome layers in ab plane, coupled antiferromagnetically. It should be noted that the layers exhibit a canted ferromagnetic spin arrangement. Applying an external magnetic field triggers a metamagnetic transition in both compounds (BC ≈ 0.8 T) [49,50,51]. Many research groups contributed to the studies of the microscopic magnetic structure of the synthetic francisites. These fruitful structural, electromagnetic, and optical studies combined with the theoretical calculations resulted in discoveries of some interesting features of the francisite-like compounds [49,50,51,52,53,54,55,56,57,58,59,60,61]. Thus, a structural transition was detected in Cu3Bi(SeO3)2O2Cl, but surprisingly it was not observed in Cu3Bi(SeO3)2O2Br. Moreover, the origin of this transition is still not clear [59,60,61]. The most recent publications suggest spin-flip-induced multiferroic behavior in Cu3Bi(SeO3)2O2Cl [57,59] and this might be a consequence of a structural transformation. However, this requires further clarification.
The compounds of composition Cu3M(SeO3)2O2X (M = La–Lu, X = Cl, Br) may be divided into two groups: the first one containing nonmagnetic rare earth ions such as Y3+, La3+, Eu3+, and Lu3+, and the second one featuring magnetic M3+ ions. Representatives of the first group demonstrate magnetic behavior similar to that of Cu3Bi(SeO3)2O2X (X = Cl, Br) [32,33,35]. χ(T) and M(H) curves typical for francisite-like compounds with non-magnetic rare earth ions are shown in Figure 17. The presence of two magnetic ions in one crystal structure leads to a much more complicated magnetic behavior. It manifests itself in the appearance of the second sharp maximum on the magnetic susceptibility curve at T < TN, as well as on the heat capacity temperature dependence curve (Figure 18). Up to now a detailed study has been undertaken for only two representatives, which are Cu3Sm(SeO3)2O2Cl [34] and Cu3Yb(SeO3)2O2Cl [37]. In both compounds, the magnetic system undergoes the spin-reorientation transition influenced by the rare earth ion’s magnetic moment. Exchange paths in the samarium compound are shown in Figure 19.
Thus, a series of isostructural compounds have similar magnetic properties, which may be described in terms of a quasi-2D model. In all these compounds an antiferromagnetic transition in the copper subsystem occurs at TN ≈ 31–41 K. In case of Cu3M(SeO3)2O2Cl (M = Y, La, Eu, Lu) it can be seen that the linear decrease of the ionic radii of M3+ leads to the decrease of the unit cell parameters (Table 1), as well as to the increase of TN and the critical field of metamagnetic transition (Table 2). Further research in this field will enable establishing the relationships between the chemical composition, the crystal structure, and the magnetic properties of compounds in this family.

2.5. Ilinskite Like Compounds MCu5(SeO3)2O2Cl3

To the best of our knowledge, the most recently described group of selenite halide d-metal compounds for which structural and physical properties characterization was reported, are copper compounds with alkali metals MCu5(SeO3)2O2Cl3 M = Na and K [38,39,62,63] and two isostructural halides Na2Cu7(SeO3)4O2X4 X = Cl and Br [40,64,65] while the chloride is only magnetically characterized (compounds 31–33 in Table 1). The first group comprises compounds derived from the naturally occurring mineral ilinskite NaCu5(SeO3)2O2Cl3 [62]. There were obtained synthetic compounds with the same composition and structure for sodium and potassium [38,39,63]. The four copper atoms in the structure may be divided in the two types of planar coordination: [CuO4] and [CuO3Cl] (Figure 20a). In NaCu5(SeO3)2O2Cl3 and KCu5(SeO3)2O2Cl3 these plaquettes form well-defined layers in the bc plane. The layers are bridged by SeO32– groups and interleaved by the M+ ions. The authors of Ref. [36] reported that the pure sodium compound is less stable due to the alkali metal ion size effect. Sodium atoms have five nearest neighbors [(O + 2Cl) + 2Cl] whereas potassium is surrounded by five oxygen and four chlorine atoms. In both compounds there is a group of Cu–Cu distances in the range of 2.85–3.28 Å. The magnetic behavior of KCu5(SeO3)2O2Cl3 was studied in [63] and may be described by a model of a system built by corner-sharing Cu4 tetrahedra. A part of the Cu sublattice with shortest distances is given in Figure 20b.
In spite of the fact that the main motifs of the ilinskite structure are in general similar to those of other copper selenite halides, magnetic behavior of KCu5(SeO3)2O2Cl3 is drastically different [63]. The χ(T) curve shows broad maximum at T ≈ 50 K, but there is no evidence for any long-range order down to 2 K (Figure 21a). The absence of a λ-type anomaly on the Cp(T) curve confirms this (Figure 21b). The negative value of the Curie–Weiss temperature reflects the predominant antiferromagnetic interactions in the magnetic copper sublattice.
Application of DFT calculations combined with thermodynamic measurements resulted in a suggestion of a possible microscopic model of spin interactions in KCu5(SeO3)2O2Cl3. The magnetic structure can be interpreted as layers of corner-sharing tetrahedra Cu4 with numerous magnetic interactions between copper atoms. Most of the exchange couplings take place between nearest neighbors, and this would lead to a strong frustration effect, but the influence of non-magnetic SeO32– groups results in the reduction of frustration. Dissimilar interactions render the system quasi-1D. Thus, the magnetic behavior can be described in terms of non-frustrated one-dimensional units (spin ladders or tubes) with only weak and frustrated couplings between these units.

2.6. Compound Na2Cu7(SeO3)4O2Cl4

Compound Na2Cu7(SeO3)4O2Cl4 [40,65] is an example of a unique structure. There are four independent Cu crystallographic sites. These atoms form heptanuclear clusters (Figure 22 left panel). The Cu–Cu distances in the clusters are in the range of 2.96–3.25 Å. Every cluster is formed by the two tetrahedra connected by a common vertex. Copper atoms in the vertices have the square planar surrounding [CuO4], two atoms in tetrahedra have the planar coordination [CuO3Cl] and finally one copper atom is surrounded by four oxygen atoms and one chlorine atoms forming [CuO4Cl] triangular bipyramid. Na2Cu7(SeO3)4O2Cl4 displays a three-dimensional network built by isolated [Cu7O14Cl4]18− clusters connected by SeO32− groups (right panel of Figure 22), in which Na+ ions are located inside the vacancies. The same structure was found for the bromide analog Na2Cu7(SeO3)4O2Br4 [64] taking into account that Cu–O and Na–O distances are elongated compared to the chloride compound.
In Na2Cu7(SeO3)4O2Cl4, antiferromagnetic long-range ordering takes place below TN ≈ 5 K. The Curie–Weiss temperature value is indicative of predominant antiferromagnetic interactions between the copper atoms. Field dependent magnetization measurements data specify the spin-flip magnetic transition. The presence of a hysteresis loop may be attributed to the ferromagnetic ground state. A magnetization plateau was observed in the magnetization curve, corresponding likely to the 3/7 magnetization plateau suggested by theoretical prediction in a reverse triangular spin-bipyramid. This data allowed proposal of a geometrical model for possible spin arrangements in magnetic sublattice.
The existence of a group with an odd number of possible spin interactions should lead to the spin-frustration effect within such groups. For Na2Cu7(SeO3)4O2Cl4 this effect was indeed found [40]. A frustration effect was evaluated using the empirical frustration parameter f:
f   = | Θ C W | T N
The value of f > 10 indicates a strong frustration effect. For the above-mentioned compound this value is calculated to be 13.6, and it is comparable with those calculated for similar known compounds with a magnetic frustration.

3. Conclusions and Outlook

We have attempted to show that interesting magnetic interactions in 3-D metal selenite halides appear due to the influence of lone pairs of SeO32– groups and the affinity of halide ions to bind with metal ions instead of selenium. In addition to the described compounds there are a lot of selenite halides with promising crystal structures. Some of such compounds were mentioned by us previously. For example, the compound Cu5(SeO3)4Br2 [64] (Table 3) shows dimorphism and crystallizes in α form which is isostructural to Cu5(SeO3)4Cl2 [22] whereas β form is close to Co5(SeO3)4X2 X = Cl, Br [23] and Ni5(SeO3)4Br2 [25]. Both α and β polymorphs show 3-D Cu–Cu frameworks with Cu–Cu distances 3.16–3.61 and 3.10–3.44 Å, respectively. Such values suggest the magnetic exchange between copper ions in Cu5(SeO3)4Br2. A similar situation may be observed in case of Na2Cu7(SeO3)4O2Br4 [64] (Table 3), an analog of the frustrated chloride compound [40]. Na2Cu7(SeO3)4O2Br4 demonstrates the same isolated [Cu7O14Br4]18− clusters with Cu–Cu distances 2.96–3.27 Å close to those in Na2Cu7(SeO3)4O2Cl4 [40].
Another alkali metal copper selenite chloride K[Cu3O](SeO3)2Cl [65] possesses a very interesting, unprecedented structure (Table 3). The unit cell of K[Cu3O](SeO3)2Cl contains three Cu2+ ions, two in [CuO4] plane coordination and squeezed tetrahedra and one [CuO4Cl] polyhedron. Copper atoms are situated in the range 2.94–3.35 Å and form layers in the structure of K[Cu3O](SeO3)2Cl. The layer may be presented as formed by edge shared pairs of Cu4 tetrahedrons which are connected into the layer in ab plane of the crystal structure Figure 23. Distances between copper atoms from different layers are about 6 Å. The system of magnetic exchange of vertex shared tetrahedra was studied in [63]. Compound K[Cu3O](SeO3)2Cl possess different type of tetrahedra connection but it may be expected that magnetic exchange will be found in this compound.
In addition to the listed compounds, some other selenite halide 3d-metals have been described in the literature and structurally characterized. These are Cu3(SeO3)2Cl2 [15,66], Cu5O2(SeO3)2Cl2 [67,68,69], Cu7O2(SeO3)2Cl6 [70], Cu9O2(SeO3)4Cl6, [71,72,73], CuICuII4O(SeO3)Cl5 [74], Co4(SeO3)3Cl2 [75], and the diselenite cobalt compound Co3(Se2O5)2Cl2 [75]. Symmetries and cell constants for these compounds are listed in Table 3. As follows from the listed data these compounds frequently form polymorphs and some of them were found in nature as copper minerals Georgbokiite [67,68,88], Parageorgbokiite [69], Nicksobolevite [70], and Chloromenite [72] while in the laboratory conditions other crystal polymorphs may be obtained. These compounds, if they will be obtained in the phase-pure form or as crystals suitable for measurements, appear as good objects for metal–metal ions magnetic exchange studies. The majority of the listed compounds show M–M distances in the range of 2.9–3.4 Å. Taking into account these distances, a complex nature of the magnetic exchange in the compounds under consideration may be expected. From the crystallographic point of view, the most attractive compounds are layered monoclinic C2/m modifications of Cu3(SeO3)2Cl2 [66] and cobalt diselenite chloride Co3(Se2O5)2Cl2 [75], where chains of [CoO4Cl2] octahedra are divided by diselenite Se2O52– groups in the crystal structure Figure 24. The compound CuICuII4O(SeO3)Cl5 [74] is a good example of a structure where Cu2+ ions form complex chains in the crystal structure separated by chains constructed by [CuICl3]2– triangles, as shown in Figure 25.
As a 3d transition metal, zinc forms a selenite chloride known as mineral Sophiite with the composition Zn2(SeO3)Cl2 [89] and its synthetic polymorph [90] but due to its 3d10 electron configuration it cannot possess magnetic properties.
Recently, a new tellurite-selenite chloride of iron with the composition Fe[(Te1.5Se0.5)O5]Cl has been prepared and structurally characterized [91]. The composition of this compound is close to that of FeTe2O5Cl [13] that shows a unique structure and possesses interesting magnetic properties, see for example [92,93,94,95]. The mixed selenite tellurite chloride of iron (III) shows the different structure [91] which is layered and contains dimers of the [FeO5Cl] octahedra, linked via common O–O edges, and mixed [Te3SeO10]4– tetramers. The Fe–Fe distance in the [Fe2O8Cl2] dimers is 3.30 Å and suggests some kind of magnetic exchange at least between Fe–Fe ions in these groups.
In addition to the above-mentioned compounds and compositions, some more complex mixed cation selenite halides have been described in the literature and may attract attention due to the possible low dimensional magnetism of 3d metal ions sublattice (Table 3 substances 15–29) [76,77,78,79,80,81,82,83,84,85,86,87]. Some of them were found in nature as minerals [80,81,82]. Among the listed compounds, the compounds of Mn2+ MnSm(SeO3)2Cl [79] and MnBi(SeO3)2Cl [84] deserve mentioning. The magnetic behavior of Mn2+ ions with S = 5/2 as well as of chromium (III) compounds in selenite halides was not described at all. To the best of our knowledge even selenites of chromium are less explored than the iron analogues. According to the literature data [96] diselenites M2(Se2O5)3 M = Fe, Cr crystallize in the same structures and behave as three-dimensional isotropic antiferromagnets with the weak antiferromagnetic coupling of M3+ ions at relatively high temperatures, but at low temperature Fe2(Se2O5)3 possesses a long range AF order while Cr2(Se2O5)3 does not, according to [96].
As was mentioned in the introduction, selenites (selenium (IV) compounds) are strong oxidants. The compound Pb2VO2(SeO3)2Cl (Table 3 compound 28) [86] is the only example of vanadium selenite halides, but in this compound the V atom is in the oxidation state +5 and has no valence electrons and could not display any magnetic interaction. For this reason, there is no data about the selenite halide compounds with V or Ti in low oxidation states, as well as with Fe2+ or Mn3+. At the same time, an anhydrous selenite of Mn(II) and Mn(III), namely, Mn3O(SeO3)3 is known [97] suggesting the possibility to obtain manganese selenite halides. As it was demonstrated for Bi compounds with the francisite type of structures, the properties may be varied within one structure type by changing the halide ion (chloride for bromide). This serves as an attractive approach to find new compounds because bromides may sometimes show different structures and magnetic properties compared to chlorides, as evidenced for example by Ni5(SeO3)4Cl2 and Ni5(SeO3)4Br2 [24,25]. It is obvious that selenite iodides are difficult to prepare due to the possible facile oxidation of the iodide ions, but the existence of Pb3(SeO3)2I2 [98] and Cu3Bi(SeO3)2O2I [31] demonstrates the possibility to find proper synthesis conditions and therefore opens further opportunities for future research.
The existence of the wide range of compounds for which it is possible to expect low-dimensional magnetic interactions gives reason for chemists to develop synthetic methods suitable for the preparation of these substances in the phase-pure form or as large single crystals.
In conclusion, we would like to underline again that 3d transition metal selenites, as well as closely related tellurites, are a promising and attractive family of inorganic compounds with rich crystal chemistry and possible occurrence of low-dimensional magnetic subsystems with a wide variety of topologies.

Acknowledgments

This work was supported by the Russian Foundation for Basic Research (RFBR) (Grants 16-03-00463a and 18-52-52005MHTa). S.S., University of Antwerp, and R.A.G. are acknowledged for discussion of the manuscript.

Author Contributions

All authors wrote parts of this work. P.S.B. coordinated the project.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Projection of the Cu5(SeO3)4Cl2 crystal structure on ab plane according to [22]. Copper polyhedra are shown.
Figure 1. Projection of the Cu5(SeO3)4Cl2 crystal structure on ab plane according to [22]. Copper polyhedra are shown.
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Figure 2. The Ni5(SeO3)4Br2 framework crystal structure (a) and Ni5(SeO3)4Cl2 structure (b). Coordination surroundings of the Ni atoms are depicted at the bottom part of the pictures.
Figure 2. The Ni5(SeO3)4Br2 framework crystal structure (a) and Ni5(SeO3)4Cl2 structure (b). Coordination surroundings of the Ni atoms are depicted at the bottom part of the pictures.
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Figure 3. Layers of [SrO6Cl3] polyhedral connected by [MO4Cl2] octahedra and SeO32– groups in the crystal structure of Sr2Co(SeO3)2Cl2 [26].
Figure 3. Layers of [SrO6Cl3] polyhedral connected by [MO4Cl2] octahedra and SeO32– groups in the crystal structure of Sr2Co(SeO3)2Cl2 [26].
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Figure 4. Crystal structure of SrCu2(SeO3)2Cl2 [27]. The [Cu2O4Cl2] dimer is shown separately at the bottom part of the figure.
Figure 4. Crystal structure of SrCu2(SeO3)2Cl2 [27]. The [Cu2O4Cl2] dimer is shown separately at the bottom part of the figure.
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Figure 5. Copper polyhedra in the crystal structures of PbCu2(SeO3)2Cl (a) and PbCu2(TeO3)2Cl (b).
Figure 5. Copper polyhedra in the crystal structures of PbCu2(SeO3)2Cl (a) and PbCu2(TeO3)2Cl (b).
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Figure 6. Magnetic susceptibility measured for CaCu2(SeO3)2Cl2 (circles) and the uniform spin-1/2 chain model fit (dashed line). Heisenberg chain (solid line) and impurity (dotted line) contributions to the fitted curve are shown. The inset shows the high-field magnetization curve (squares) with a linear fit (dashed line) Reprinted figure with permission from [28] Copyright (2011) by the American Physical Society.
Figure 6. Magnetic susceptibility measured for CaCu2(SeO3)2Cl2 (circles) and the uniform spin-1/2 chain model fit (dashed line). Heisenberg chain (solid line) and impurity (dotted line) contributions to the fitted curve are shown. The inset shows the high-field magnetization curve (squares) with a linear fit (dashed line) Reprinted figure with permission from [28] Copyright (2011) by the American Physical Society.
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Figure 7. Spin model for CaCu2(SeO3)2Cl2. Filled and empty circles show the Cu(1) and Cu(2) positions, respectively. Bold lines and circles denote atoms in the front plane, whereas gray circles and shaded lines correspond to the atoms lying in the back plane. The planes are connected by Jic1 couplings only. A closed loop (bold line) having an odd number of AFM couplings indicates that the spin model is frustrated. The unit cell is depicted by the gray rectangle. Reprinted figure with permission from [28] Copyright (2011) by the American Physical Society.
Figure 7. Spin model for CaCu2(SeO3)2Cl2. Filled and empty circles show the Cu(1) and Cu(2) positions, respectively. Bold lines and circles denote atoms in the front plane, whereas gray circles and shaded lines correspond to the atoms lying in the back plane. The planes are connected by Jic1 couplings only. A closed loop (bold line) having an odd number of AFM couplings indicates that the spin model is frustrated. The unit cell is depicted by the gray rectangle. Reprinted figure with permission from [28] Copyright (2011) by the American Physical Society.
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Figure 8. (left) Crystal structure of CaCu2(SeO3)2Cl2. The structural chains run along [10¯1] (not shown), whereas the magnetic chains range along ∼[201], as shown by bold (red) lines. Small unlabeled spheres denote O atoms. (right) Local environment of Cu(1) and Cu(2). The magnetically active Cu(1)O4 and Cu(2)O2Cl2 plaquettes are highlighted. Reprinted figure with permission from [28] Copyright (2011) by the American Physical Society.
Figure 8. (left) Crystal structure of CaCu2(SeO3)2Cl2. The structural chains run along [10¯1] (not shown), whereas the magnetic chains range along ∼[201], as shown by bold (red) lines. Small unlabeled spheres denote O atoms. (right) Local environment of Cu(1) and Cu(2). The magnetically active Cu(1)O4 and Cu(2)O2Cl2 plaquettes are highlighted. Reprinted figure with permission from [28] Copyright (2011) by the American Physical Society.
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Figure 9. Magnetic susceptibility of PbCu2(SeO3)2Cl2 (circles) and the fit (solid line) using a uniform Heisenberg chain model with J1 = 160 K, g = 1.97, and the temperature independent contribution χ0 = 2 × 10−5 emu (mol Cu)−1. The kink at 13 K indicates long-range magnetic ordering. Inset: field dependence of χ(T). The strong dependence of χ on the magnetic field above TN is an extrinsic effect (ferromagnetic impurity). Reproduced from Ref. [29] with permission from the Royal Society of Chemistry.
Figure 9. Magnetic susceptibility of PbCu2(SeO3)2Cl2 (circles) and the fit (solid line) using a uniform Heisenberg chain model with J1 = 160 K, g = 1.97, and the temperature independent contribution χ0 = 2 × 10−5 emu (mol Cu)−1. The kink at 13 K indicates long-range magnetic ordering. Inset: field dependence of χ(T). The strong dependence of χ on the magnetic field above TN is an extrinsic effect (ferromagnetic impurity). Reproduced from Ref. [29] with permission from the Royal Society of Chemistry.
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Figure 10. Crystal structure of PbCu2(SeO3)2Cl2. (a) The unit cell. Magnetic Cu(1)O4 and Cu(2)O2Cl2 plaquettes are shown as light and dark filled polyhedra. Se–O bonds and Pb atoms are denoted by lines and large black spheres, respectively. (b) The coordination polyhedron of Pb. (c) Structural chains, formed by cornersharing Cu(1)O4 distorted plaquettes and Cu(2)O4Cl2 octahedra (empty octahedra, with filled Cu(2)O2Cl2 plaquettes; long Cu(2)-O contacts are shown with thick dashed lines). (d) Magnetic chains. The nearest-neighbor superexchange J1 runs via Cu(1)–O⋯O–Cu(2) paths, along the edge of a SeO3 pyramid (Se–O bonds are shown with lines). Reproduced from Ref. [29] with permission from the Royal Society of Chemistry.
Figure 10. Crystal structure of PbCu2(SeO3)2Cl2. (a) The unit cell. Magnetic Cu(1)O4 and Cu(2)O2Cl2 plaquettes are shown as light and dark filled polyhedra. Se–O bonds and Pb atoms are denoted by lines and large black spheres, respectively. (b) The coordination polyhedron of Pb. (c) Structural chains, formed by cornersharing Cu(1)O4 distorted plaquettes and Cu(2)O4Cl2 octahedra (empty octahedra, with filled Cu(2)O2Cl2 plaquettes; long Cu(2)-O contacts are shown with thick dashed lines). (d) Magnetic chains. The nearest-neighbor superexchange J1 runs via Cu(1)–O⋯O–Cu(2) paths, along the edge of a SeO3 pyramid (Se–O bonds are shown with lines). Reproduced from Ref. [29] with permission from the Royal Society of Chemistry.
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Figure 11. The view on [FeO6] octahedra chains decorated by Bi polyhedra and SeO32– groups in the crystal structure of Bi2Fe(SeO3)2OCl3.
Figure 11. The view on [FeO6] octahedra chains decorated by Bi polyhedra and SeO32– groups in the crystal structure of Bi2Fe(SeO3)2OCl3.
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Figure 12. Temperature dependence of magnetic susceptibility in Bi2Fe(SeO3)2OCl3. The solid line represents the fit in a Heisenberg S = ∞ antiferromagnetic chain model. The inset represents the temperature dependence of specific heat on the C/T vs. T2 scale. Reprinted with permission from [30] Copyright (2014) American Chemical Society.
Figure 12. Temperature dependence of magnetic susceptibility in Bi2Fe(SeO3)2OCl3. The solid line represents the fit in a Heisenberg S = ∞ antiferromagnetic chain model. The inset represents the temperature dependence of specific heat on the C/T vs. T2 scale. Reprinted with permission from [30] Copyright (2014) American Chemical Society.
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Figure 13. Exchange paths for various magnetic interactions J∥ (in black), J∥′ (in cyan), and J﬩ (in gold) in Bi2Fe(SeO3)2OCl3. Reprinted with permission from [30]. Copyright (2014) American Chemical Society.
Figure 13. Exchange paths for various magnetic interactions J∥ (in black), J∥′ (in cyan), and J﬩ (in gold) in Bi2Fe(SeO3)2OCl3. Reprinted with permission from [30]. Copyright (2014) American Chemical Society.
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Figure 14. The crystal structure of Cu3Bi(SeO3)2Cl. [BiO8] distorted cubes are shown.
Figure 14. The crystal structure of Cu3Bi(SeO3)2Cl. [BiO8] distorted cubes are shown.
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Figure 15. The copper-oxygen net in the crystal structure of Francisite-like compounds crystal structure shown in different direction. Cu–Cu contacts are marked as green. The right part of the figure shows the buckled nature of the kagome net.
Figure 15. The copper-oxygen net in the crystal structure of Francisite-like compounds crystal structure shown in different direction. Cu–Cu contacts are marked as green. The right part of the figure shows the buckled nature of the kagome net.
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Figure 16. The variation of Cu1-Cu1, Cu1-Cu2, and ∠Cu1-Cu2-Cu1 angles as function of ionic REM radii for Cu3Ln(SeO3)2O2Cl compounds.
Figure 16. The variation of Cu1-Cu1, Cu1-Cu2, and ∠Cu1-Cu2-Cu1 angles as function of ionic REM radii for Cu3Ln(SeO3)2O2Cl compounds.
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Figure 17. (a) The temperature dependence of the magnetic susceptibility of Cu3Y(SeO3)2O2Cl at applied field strengths of 1–9 T, with a field step of 1 T. The inset represents the temperature dependence of the magnetic susceptibility at 0.1 T along with the Curie constant as a function of temperature. The dotted line represents an extrapolation of the Curie-Weiss type dependence from the high temperature region. (b) The field dependence of the magnetization in Cu3Y(SeO3)2O2Cl over the temperature range of 5–30 K in steps of 5 K. The inset shows the field dependence of the magnetization at 2 K and its field derivative. Reprinted figure with permission from [32] Copyright (2014) by the American Physical Society.
Figure 17. (a) The temperature dependence of the magnetic susceptibility of Cu3Y(SeO3)2O2Cl at applied field strengths of 1–9 T, with a field step of 1 T. The inset represents the temperature dependence of the magnetic susceptibility at 0.1 T along with the Curie constant as a function of temperature. The dotted line represents an extrapolation of the Curie-Weiss type dependence from the high temperature region. (b) The field dependence of the magnetization in Cu3Y(SeO3)2O2Cl over the temperature range of 5–30 K in steps of 5 K. The inset shows the field dependence of the magnetization at 2 K and its field derivative. Reprinted figure with permission from [32] Copyright (2014) by the American Physical Society.
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Figure 18. (a) The temperature dependences of magnetic susceptibility in Cu3Sm(SeO3)2O2Cl in ZFC (squares) and FC (circles) regimes taken at B = 0.02 T. Inset: The ZFC curve in the range 2–320 K and its fit (dash). Also shown is the temperature dependence of Curie constant C = (χ − χ0)(T − Θ). (b) The temperature dependences of specific heat in selected magnetic fields for Cu3Sm(SeO3)2O2Cl. Inset: Temperature dependence of magnetic entropy Smag Reprinted figures with permission from [34]. Copyright (2016) by the American Physical Society.
Figure 18. (a) The temperature dependences of magnetic susceptibility in Cu3Sm(SeO3)2O2Cl in ZFC (squares) and FC (circles) regimes taken at B = 0.02 T. Inset: The ZFC curve in the range 2–320 K and its fit (dash). Also shown is the temperature dependence of Curie constant C = (χ − χ0)(T − Θ). (b) The temperature dependences of specific heat in selected magnetic fields for Cu3Sm(SeO3)2O2Cl. Inset: Temperature dependence of magnetic entropy Smag Reprinted figures with permission from [34]. Copyright (2016) by the American Physical Society.
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Figure 19. The exchange paths for various magnetic interactions in Cu3Sm(SeO3)2O2Cl. Reprinted figure with permission from [34]. Copyright (2016) by the American Physical Society.
Figure 19. The exchange paths for various magnetic interactions in Cu3Sm(SeO3)2O2Cl. Reprinted figure with permission from [34]. Copyright (2016) by the American Physical Society.
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Figure 20. The crystal structure of NaCu5(SeO3)2O2Cl3. Two types of [CuO4] and [CuO3Cl] plaquettes are shown (a); A fragment of the corner shared Cu4 tetrahedra sublattice layer in the crystal structure of ilinskite like compounds (b).
Figure 20. The crystal structure of NaCu5(SeO3)2O2Cl3. Two types of [CuO4] and [CuO3Cl] plaquettes are shown (a); A fragment of the corner shared Cu4 tetrahedra sublattice layer in the crystal structure of ilinskite like compounds (b).
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Figure 21. (a) Magnetic susceptibility χ(T) for KCu5O2(SeO3)2Cl3 obtained at different values of the external magnetic field in the field-cooling regime. The inset shows the Curie–Weiss approximation in the 100–380 K temperature range with the parameters θ = 60 K and C = 2.3 emu K/mol, as denoted by the green line. (b) The magnetization curve measured at T = 1.5 K. (left) Temperature dependence of the specific heat, Cp(T)/T, for KCu5O2(SeO3)2Cl3 measured in zero field. Reproduced from [63] under Creative Commons Attribution 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.
Figure 21. (a) Magnetic susceptibility χ(T) for KCu5O2(SeO3)2Cl3 obtained at different values of the external magnetic field in the field-cooling regime. The inset shows the Curie–Weiss approximation in the 100–380 K temperature range with the parameters θ = 60 K and C = 2.3 emu K/mol, as denoted by the green line. (b) The magnetization curve measured at T = 1.5 K. (left) Temperature dependence of the specific heat, Cp(T)/T, for KCu5O2(SeO3)2Cl3 measured in zero field. Reproduced from [63] under Creative Commons Attribution 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.
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Figure 22. [Cu7O14Cl4]18− cluster (left) and the copper polyhedra with SeO32– groups in the crystal structure of Na2Cu7(SeO3)4O2Cl4 (right) [40].
Figure 22. [Cu7O14Cl4]18− cluster (left) and the copper polyhedra with SeO32– groups in the crystal structure of Na2Cu7(SeO3)4O2Cl4 (right) [40].
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Figure 23. The presentation of the copper-oxygen layer decorated by SeO32– groups in different directions in the crystal structure of the K[Cu3O](SeO3)2Cl [65]. Blue lines represent Cu–Cu contacts.
Figure 23. The presentation of the copper-oxygen layer decorated by SeO32– groups in different directions in the crystal structure of the K[Cu3O](SeO3)2Cl [65]. Blue lines represent Cu–Cu contacts.
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Figure 24. The chain built by [CoO4Cl2] octahedra in the crystal structure of Co3(Se2O5)Cl2 (left) and view on chains interconnected by diselenite groups (right) [75].
Figure 24. The chain built by [CoO4Cl2] octahedra in the crystal structure of Co3(Se2O5)Cl2 (left) and view on chains interconnected by diselenite groups (right) [75].
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Figure 25. The complex chains of Cu2+ ions in the structure of CuICuII4O(SeO3)Cl5 [74] separated by [CuICl3]2– groups and decorated by SeO32– groups.
Figure 25. The complex chains of Cu2+ ions in the structure of CuICuII4O(SeO3)Cl5 [74] separated by [CuICl3]2– groups and decorated by SeO32– groups.
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Table
Table 1. Compositions and crystallographic data for transition metals selenite halides with described magnetic properties.
Table 1. Compositions and crystallographic data for transition metals selenite halides with described magnetic properties.
#CompositionSpace GroupZ Cell Constants Ref.
a, Åb, Åc, ÅAngles, °
1Cu5(SeO3)4Cl2P21/c210.9104(8)8.3134(6)7.5490(6)β = 90.715(6)[22]
2Co5(SeO3)4Cl2P-116.4935(8)7.7288(8)7.7443(10)α = 66.051(11)[23]
β = 73.610(11)
γ = 81.268(9)
3Co5(SeO3)4Br2P-116.4897(9)7.7574(10)7.7552(10)α = 66.850(13)[23]
β = 73.960(12)
γ = 81.350(11)
4Ni5(SeO3)4Cl2P-128.076(2)9.288(2)9.376(2)α = 101.97(3)[24]
β = 105.60(3)
γ = 91.83(3)
5Ni5(SeO3)4Br2P-116.430(3)7.632(3)7.658(3)α = 68.017(16)[25]
β = 74.181(16)
γ = 81.465(19)
6Sr2Co(SeO3)2Cl2P21/n25.3400(10)6.4279(6)12.3220(10)β = 92.440(10)[26]
7Sr2Ni(SeO3)2Cl2P21/n25.3254(11)6.4363(13)12.197(2)β = 92.53(3)[26]
8Sr2Cu(SeO3)2Cl2P21/n25.22996(3)6.50528(4)12.34518(7)β = 91.3643(2)[27]
9CaCu2(SeO3)2Cl2C2/c412.759(3)9.0450(18)6.9770(14)β = 91.03(3)[28]
10SrCu2(SeO3)2Cl2P2127.1630(14)7.2070(14)8.0430(16)β = 95.92(3)[27]
11PbCu2(SeO3)2Cl2C2/c413.056(1)9.5567(9)6.9006(6)β = 90.529(7)[29]
12Bi2Fe(SeO3)2OCl3P21/m28.570(2)7.137(2)8.604(2)β = 107.090(3)[30]
13Cu3Bi(SeO3)2O2ClPmmn26.3540(4)9.6350(5)7.2330(4) [31]
14Cu3Bi(SeO3)2O2BrPmmn26.3900(3)9.6940(4)7.2870(3) [31]
15Cu3Bi(SeO3)2O2IPmmn26.4360(2)9.7510(4)7.3770(3) [31]
16Cu3Y(SeO3)2O2ClPmmn26.2991(1)9.4411(1)6.9724(1) [32]
17Cu3La(SeO3)2O2ClPmmn26.39407(18)9.7310(3)7.1547(2) [33]
18Cu3Nd(SeO3)2O2ClPmmn26.37775(10)9.62685(16)7.09341(11) [31]
19Cu3Sm(SeO3)2O2ClPmmn26.34616(4)9.56090(7)7.04377(5) [34]
20Cu3Eu(SeO3)2O2ClPmmn26.3384 (1)9.5341(2)7.0273(1) [35]
21Cu3Gd(SeO3)2O2ClPmmn26.3220(6)9.501(1)7.0202(8 [36]
22Cu3Dy(SeO3)2O2ClPmmn26.313(1)9.465(2)6.987(2) [36]
23Cu3Ho(SeO3)2O2ClPmmn26.2999(6)9.440(1)6.9723(8) [36]
24Cu3Er(SeO3)2O2ClPmmn26.299(1)9.432(3)6.967(2) [36]
25Cu3Yb(SeO3)2O2ClPmmn26.28278(3)9.39486(5)6.93291(3) [37]
26Cu3Lu(SeO3)2O2ClPmmn26.2681(1)9.3756(2)6.9326(1) [35]
27Cu3La(SeO3)2O2BrPmmn26.40071(5)9.75675(7)7.17800(5) [33]
28Cu3Nd(SeO3)2O2BrPmmn26.382(2)9.698(3)7.091(2) [36]
29Cu3Sm(SeO3)2O2BrPmmn26.348(1)9.581(2)7.079(2) [36]
30Cu3Gd(SeO3)2O2BrPmmn26.337(1)9.5515(8)7.0540(9) [36]
31NaCu5(SeO3)2O2Cl3Pnma417.769(7)6.448(3)10.522(4) [38]
32КCu5(SeO3)2O2Cl3Pnma418.1691(6)6.4483(2)10.5684(4) [39]
33Na2Cu7(SeO3)4O2Cl4P-117.446(2)8.349(3)9.137(3)α = 110.335(7)[40]
β = 106.166(3)
γ = 105.161(7)
Table
Table 2. Néel temperatures of antiferromagnetic ordering and metamagnetic transitions critical field values for Cu3M(SeO3)2O2Cl (M = Y, La, Eu, Lu) and Cu3M(SeO3)2O2Br (M = Bi, La).
Table 2. Néel temperatures of antiferromagnetic ordering and metamagnetic transitions critical field values for Cu3M(SeO3)2O2Cl (M = Y, La, Eu, Lu) and Cu3M(SeO3)2O2Br (M = Bi, La).
TN, KBC, TRef.
Cu3Bi(SeO3)2O2Br27.40.8[49]
Cu3La(SeO3)2O2Cl31.22.4[33]
Cu3La(SeO3)2O2Br34.32.45[33]
Cu3Eu(SeO3)2O2Cl362.6[35]
Cu3Y(SeO3)2O2Cl36.32.6[32]
Cu3Lu(SeO3)2O2Cl383.0[35]
Table
Table 3. Compositions and crystallographic data for transition metals selenite halides with possible magnetic exchange between d-metal ions.
Table 3. Compositions and crystallographic data for transition metals selenite halides with possible magnetic exchange between d-metal ions.
#CompositionSpace GroupZ Cell Constants Ref.
a, Åb, Åc, ÅAngles, °
1α-Cu5(SeO3)4Br2P21/c211.1089(18)8.3233(13)7.5668(12)β = 90.893(3)[64]
2β-Cu5(SeO3)4Br2P-116.2096(13)7.8553(16)7.9006(17)α = 65.538(6)[64]
β = 83.111(7)
γ = 75.291(7)
3Na2Cu7(SeO3)4O2Br4P-147.7657(3)8.3750(3)9.2626(4)α = 110.227(2)[64]
β = 104.897(2)
γ = 107.195(3)
4K[Cu3O](SeO3)2ClP-127.6821(5)8.1179(5)8.7836(6)α = 113.193(3)[65]
β = 108.735(4)
γ = 98.245(4)
5Cu3(SeO3)2Cl2C2/m28.9333(12)6.2164(7)7.5815(12)β = 110.238(13)[66]
6Cu3(SeO3)2Cl2P-126.1240(4)7.7880(5)8.5170(6)α = 92.755(4)[15]
β = 95.735(4)
γ = 92.853(4)
7Cu5O2(SeO3)2Cl2P21/c26.030(1)13.744(3)5.562(1)β = 95.75(1)[67,68]
8β-Cu5O2(SeO3)2Cl2P21/c25.3982(5)8.0543(8)11.1277(10)β = 99.258(2)[69]
9Cu7O2(SeO3)2Cl6P21/c410.958(9)14.483(5)10.494(14)β = 113.61(7)[70]
10Cu9O2(SeO3)4Cl6I2/m214.170(3)6.262(1)12.999(3)β = 113.05(1)[71,72]
11Cu9O2(SeO3)4CI6P21/n212.922(3)6.262(2)14.042(4)β = 112.88(2)[73]
12CuICuII4O(SeO3)Cl5P21/m29.203(3)6.232(2)9.557(3)β = 91.970(8)[74]
13Co4(SeO3)3Cl2Pnma47.9751(1)14.4048(2)9.7103(2) [75]
14Co3(Se2O5)2Cl2C2/m27.1973(10)13.9961(19)5.8334(9)β = 107.524(16)[75]
15Ba2Co(SeO3)2Cl2Pnnm26.7635(4)12.6454(7)5.3866(3) [76]
16Fe6Ca2(SeO3)9Cl4P63/m212.118(2) 12.703(4) [77]
17CoNd10(SeO3)12Cl8P2/c415.699(2)15.7002(2)19.171(2)β = 113,995(5)[78]
18CoSm(SeO3)2ClP-147.123(1)8.8895(2)12.162(2)α = 72.25(1)[79]
β = 71.27(1)
γ = 72.08(1)
19CuGd(SeO3)2ClP-147.043(4)9.096(4)12.010(7)α = 70.84(4)[79]
β = 73.01(4)
γ = 70.69(4)
20MnSm(SeO3)2ClP-127.008(2)7.241(2)8.034(2)α = 86.90(3)[79]
β = 71.57(3)
γ = 64.33(3)
21CuICuII5PbO2(SeO3)2Cl5C2/m418.468(2)6.1475(8)15.314(2)β = 119.284(2)[80]
22KCdCu7O2(SeO3)2Cl9P63/mmc28.7805(8) 15.521(2) [81]
23KPb1.5Cu6Zn(SeO3)2O2Cl10Pnnm49.132(2)19.415(4)13.213(3) [82]
24MnBi(SeO3)2ClP-127.0926(8)7.2695(6)8.0160(8)α = 88.226(4)[83,84]
β = 72.005(3)
γ = 64.560(4)
25(Pb2Cu2+9O4)(SeO3)4(Cu+Cl2)Cl5C2/m218.605(17)6.204(6)12.673(12)β = 109.869(17)[85]
26(PbCu2+5O2)(SeO3)2(Cu+Cl2)Cl3C2/m218.4956(4)6.14540(10)15.2985(4)β = 119.3111(10)[85]
27(PbxCu2+(6−x)O2)(SeO3)2(Cu+Cl2)K(1−x)Cl(4−x)C2/m115.1158(11)6.1853(4)9.2672(9)β = 95.965(5)[85]
28Pb2VO2(SeO3)2ClP2128.333(3)5.3171(16)10.710(4)β = 111.701(5)[86]
29Pb5Cu(SeO3)4Cl4C2/c424.917(3)5.5060(10)14.242(2)β = 101.770(10)[87]

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Berdonosov, P.S.; Kuznetsova, E.S.; Dolgikh, V.A. Transition Metal Selenite Halides: A Fascinating Family of Magnetic Compounds. Crystals 2018, 8, 159. https://doi.org/10.3390/cryst8040159

AMA Style

Berdonosov PS, Kuznetsova ES, Dolgikh VA. Transition Metal Selenite Halides: A Fascinating Family of Magnetic Compounds. Crystals. 2018; 8(4):159. https://doi.org/10.3390/cryst8040159

Chicago/Turabian Style

Berdonosov, Peter S., Elena S. Kuznetsova, and Valery A. Dolgikh. 2018. "Transition Metal Selenite Halides: A Fascinating Family of Magnetic Compounds" Crystals 8, no. 4: 159. https://doi.org/10.3390/cryst8040159

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