Crystals

12 pages, 1127 KiB

Open AccessArticle

Complex Disorder in Type-I Clathrates: Synthesis and Structural Characterization of A₈Ga_xSn_46−x (A = Rb, Cs; 6.9 < x < 7.5)

by Sviatoslav A. Baranets, Amanda B. Childs, Hua He and Svilen Bobev

Crystals 2020, 10(4), 298; https://doi.org/10.3390/cryst10040298 - 13 Apr 2020

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Abstract

Exploratory studies in the systems Rb–Ga–Sn and Cs–Ga–Sn yielded the cubic type-I clathrates with refined compositions Rb₈Ga_xSn_46−x and Cs₈Ga_xSn_46−x (6.9 < x < 7.5). Nearly single-phase materials with good crystallinity were obtained [...] Read more.

Exploratory studies in the systems Rb–Ga–Sn and Cs–Ga–Sn yielded the cubic type-I clathrates with refined compositions Rb₈Ga_xSn_46−x and Cs₈Ga_xSn_46−x (6.9 < x < 7.5). Nearly single-phase materials with good crystallinity were obtained from stoichiometric reactions of the elements. The structures were characterized by means of single-crystal X-ray diffraction methods. Both Rb₈Ga_xSn_46−x and Cs₈Ga_xSn_46−x represents cases, where a Group 13 element randomly substitutes a Group 14 element in the structure. The extent of Ga/Sn mixing is apparently governed by the drive of the system to achieve an optimal valence electron count, and hence, Rb₈Ga_xSn_46−x and Cs₈Ga_xSn_46−x (x ≈ 8) can be regarded as Zintl phases. This notion is supported by structure refinements on a multitude of single-crystal X-ray diffraction data, which also confirm that both types of cages in the cubic type-I structure are fully occupied by Rb and Cs atoms. The open-framework, comprised of 46 nodes per formula unit, adapts to the incorporation of nearly eight Ga atoms within the matrix of Sn, whereby small, short-range distortions result. The exact nature of these effects is still unclear, as so far, the structural variations could only be modeled as both positional and occupational disorder at one of three framework sites. Since vacancies in the structures of the binary type-I clathrates A₈Sn_46−x☐_x (A = Rb, Cs; ☐ = missing Sn atom) are also known to cause local distortions, the latter were also synthesized with the same protocols used for the synthesis of A8GaxSn46−x and structurally re-analyzed. The results from the latter studies confirm that homogeneity issues abound, and that the final structures/compositions are an intricate function of the experimental conditions. Full article

(This article belongs to the Special Issue Intermetallic)

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9 pages, 1694 KiB

Open AccessEditor’s ChoiceArticle

Helimagnetism in MnBi₂Se₄ Driven by Spin-Frustrating Interactions Between Antiferromagnetic Chains

by Judith K. Clark, Chongin Pak, Huibo Cao and Michael Shatruk

Crystals 2021, 11(3), 242; https://doi.org/10.3390/cryst11030242 - 27 Feb 2021

Cited by 10 | Viewed by 3885

Abstract

We report the magnetic properties and magnetic structure determination for a linear-chain antiferromagnet, MnBi₂Se₄. The crystal structure of this material contains chains of edge-sharing MnSe₆ octahedra separated by Bi atoms. The magnetic behavior is dominated by intrachain antiferromagnetic [...] Read more.

We report the magnetic properties and magnetic structure determination for a linear-chain antiferromagnet, MnBi₂Se₄. The crystal structure of this material contains chains of edge-sharing MnSe₆ octahedra separated by Bi atoms. The magnetic behavior is dominated by intrachain antiferromagnetic (AFM) interactions, as demonstrated by the negative Weiss constant of −74 K obtained by the Curie–Weiss fit of the paramagnetic susceptibility measured along the easy-axis magnetization direction. The relative shift of adjacent chains by one-half of the chain period causes spin frustration due to interchain AFM coupling, which leads to AFM ordering at T_N = 15 K. Neutron diffraction studies reveal that the AFM ordered state exhibits an incommensurate helimagnetic structure with the propagation vector k = (0, 0.356, 0). The Mn moments are arranged perpendicular to the chain propagation direction (the crystallographic b axis), and the turn angle around the helix is 128°. The magnetic properties of MnBi₂Se₄ are discussed in comparison to other linear-chain antiferromagnets based on ternary mixed-metal halides and chalcogenides. Full article

(This article belongs to the Special Issue Intermetallic)

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15 pages, 4801 KiB

Open AccessArticle

Nowotny Chimney Ladder Phases with Group 5 Metals: Crystal and Electronic Structure and Relations to the CrSi₂ Structure Type

by Maxim S. Likhanov, Nikita V. Sytov, Zheng Wei, Evgeny V. Dikarev and Andrei V. Shevelkov

Crystals 2020, 10(8), 670; https://doi.org/10.3390/cryst10080670 - 3 Aug 2020

Cited by 2 | Viewed by 3147

Abstract

Nowotny chimney ladder (NCL) phases are intermetallic compounds formed by transition metals and metals of groups 13 and 14. This family can be expanded by combining two p-elements from different groups with those transition metals, for which the corresponding binary NCL phases [...] Read more.

Nowotny chimney ladder (NCL) phases are intermetallic compounds formed by transition metals and metals of groups 13 and 14. This family can be expanded by combining two p-elements from different groups with those transition metals, for which the corresponding binary NCL phases are unknown. In this paper, we present three new compounds in the V-Al-Ge, Nb-Al-Ge, and Nb-Ga-Ge systems related to the TiSi₂ structure type (Sp. Gr. Fddd) obtained with the standard ampule technique. The crystal structures of the new compounds were determined using synchrotron powder X-ray diffraction data. A transition to the CrSi₂ structure type was detected upon changing the composition from VAl_0.72(2)Ge_1.28(2) to VAl_1.534(3)Ge_0.466(3). According to the 18–n rule, all the compounds are metallic conductors, which was supported by the electronic structure calculations. It was shown that the expected energy gap located above the Fermi level in the vanadium-based NCL compound collapsed into a pseudogap upon the replacement of V by Nb. Full article

(This article belongs to the Special Issue Intermetallic)

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26 pages, 7186 KiB

Open AccessReview

Spotlight on Alkali Metals: The Structural Chemistry of Alkali Metal Thallides

by Stefanie Gärtner

Crystals 2020, 10(11), 1013; https://doi.org/10.3390/cryst10111013 - 7 Nov 2020

Cited by 8 | Viewed by 3209

Abstract

Alkali metal thallides go back to the investigative works of Eduard Zintl about base metals in negative oxidation states. In 1932, he described the crystal structure of NaTl as the first representative for this class of compounds. Since then, a bunch of versatile [...] Read more.

Alkali metal thallides go back to the investigative works of Eduard Zintl about base metals in negative oxidation states. In 1932, he described the crystal structure of NaTl as the first representative for this class of compounds. Since then, a bunch of versatile crystal structures has been reported for thallium as electronegative element in intermetallic solid state compounds. For combinations of thallium with alkali metals as electropositive counterparts, a broad range of different unique structure types has been observed. Interestingly, various thallium substructures at the same or very similar valence electron concentration (VEC) are obtained. This in return emphasizes that the role of the alkali metals on structure formation goes far beyond ancillary filling atoms, which are present only due to charge balancing reasons. In this review, the alkali metals are in focus and the local surroundings of the latter are discussed in terms of their crystallographic sites in the corresponding crystal structures. Full article

(This article belongs to the Special Issue Intermetallic)

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13 pages, 1949 KiB

Open AccessArticle

Revealing the Bonding Nature in an ALnZnTe₃-Type Alkaline-Metal (A) Lanthanide (Ln) Zinc Telluride by Means of Experimental and Quantum-Chemical Techniques

by Katharina Eickmeier and Simon Steinberg

Crystals 2020, 10(10), 916; https://doi.org/10.3390/cryst10100916 - 10 Oct 2020

Cited by 10 | Viewed by 2315

Abstract

Tellurides have attracted an enormous interest in the quest for materials addressing future challenges, because many of them are at the cutting edge of basic research and technologies due to their remarkable chemical and physical properties. The key to the tailored design of [...] Read more.

Tellurides have attracted an enormous interest in the quest for materials addressing future challenges, because many of them are at the cutting edge of basic research and technologies due to their remarkable chemical and physical properties. The key to the tailored design of tellurides and their properties is a thorough understanding of their electronic structures including the bonding nature. While a unique type of bonding has been recently identified for post-transition-metal tellurides, the electronic structures of tellurides containing early and late-transition-metals have been typically understood by applying the Zintl−Klemm concept; yet, does the aforementioned formalism actually help us in understanding the electronic structures and bonding nature in such tellurides? To answer this question, we prototypically examined the electronic structure for an alkaline metal lanthanide zinc telluride, i.e., RbDyZnTe₃, by means of first-principles-based techniques. In this context, the crystal structures of RbLnZnTe₃ (Ln = Gd, Tb, Dy), which were obtained from high-temperature solid-state syntheses, were also determined for the first time by employing X-ray diffraction techniques. Full article

(This article belongs to the Special Issue Intermetallic)

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15 pages, 3365 KiB

Open AccessArticle

Bismuth Polycations Revisited: Alternative Synthesis and Electronic Structure of Bi₆Br₇, and Bonding in Main-Group Polyatomic Ions from a Direct Space Perspective

by Ekaterina A. Stroganova, Sergey I. Troyanov, Igor V. Morozov and Alexey N. Kuznetsov

Crystals 2020, 10(10), 940; https://doi.org/10.3390/cryst10100940 - 16 Oct 2020

Cited by 4 | Viewed by 3391

Abstract

A bismuth subbromide, Bi₆Br₇, was synthesized in the form of single crystals using the reaction between Bi and Hg₂Br₂ in a gradient furnace. Its crystal structure was reinvestigated by low-temperature single-crystal X-ray diffraction (Pnnm, [...] Read more.

A bismuth subbromide, Bi₆Br₇, was synthesized in the form of single crystals using the reaction between Bi and Hg₂Br₂ in a gradient furnace. Its crystal structure was reinvestigated by low-temperature single-crystal X-ray diffraction (Pnnm, a = 15.4996(6) Å, b = 23.6435(7) Å, c = 9.0231(2) Å, Z = 8, R₁ = 0.041, wR_all = 0.087). Based on the diffraction data, the structure description was revised as containing Bi₉⁵⁺ cluster polycations and ¹_∞[Bi₃Br₁₄⁵⁻] ladder-like anions. DFT calculations of band structure showed the compound to be a narrow-gap semiconductor with a band gap of ca. 1.3 eV, with the nature of the compound as ionic salt confirmed by charge density analysis. Direct-space bonding analysis based on the ELF topology and QTAIM partitioning, performed for all known homoatomic bismuth polycations, as well as isoelectronic main-group metal ions, shows patterns of localized pairwise and three-center bonding forming the frameworks of the clusters. In addition to obtaining new data, the use of highly augmented basis sets allowed us to revise and amend several previously made conclusions regarding bonding in such species. Full article

(This article belongs to the Special Issue Intermetallic)

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15 pages, 4536 KiB

Open AccessArticle

Ternary Aluminides of a New Homologous Series—CePt₂Al₂ and CePt₃Al₃: Crystal Structures and Thermal Properties

by Elena Murashova, Yuliya Morozova, Sergey Dunaev, Zhanafiya Kurenbaeva and Anna Tursina

Crystals 2020, 10(6), 465; https://doi.org/10.3390/cryst10060465 - 1 Jun 2020

Cited by 3 | Viewed by 2518

Abstract

In the process of studying the Ce–Pt–Al system, we identified CePt₂Al₂ and CePt₃Al₃, two new ternary intermetallic compounds. CePt₂Al₂ aluminide undergoes a structural phase transition from a low-temperature orthorhombic modification (of its own [...] Read more.

In the process of studying the Ce–Pt–Al system, we identified CePt₂Al₂ and CePt₃Al₃, two new ternary intermetallic compounds. CePt₂Al₂ aluminide undergoes a structural phase transition from a low-temperature orthorhombic modification (of its own structure type, Cmme, a = 5.84138(2) Å, b = 6.39099(3) Å, c = 10.11611(5) Å) to a high-temperature tetragonal modification (CaBe₂Ge₂ type, P4/nmm, a = 4.3637(9) Å, c = 10.0925(14) Å) at 280(1) °C. CePt₃Al₃ crystallizes with a new type of structure (Cmme, a = 6.36548(6) Å, b = 5.78301(6) Å, c = 13.36245(19) Å) built of structural units of low-temperature orthorhombic CePt₂Al₂-type and CsCl-type. Full article

(This article belongs to the Special Issue Intermetallic)

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14 pages, 5682 KiB

Open AccessArticle

First Principles Density Functional Theory Prediction of the Crystal Structure and the Elastic Properties of Mo₂ZrB₂ and Mo₂HfB₂

by Rachid Stefan Touzani and Manja Krüger

Crystals 2020, 10(10), 865; https://doi.org/10.3390/cryst10100865 - 24 Sep 2020

Cited by 4 | Viewed by 3071

Abstract

The Molybdenum rich ternary alloys Mo-M-B (M = Zr, Hf) contain, next to the Mo solid solution (bcc Mo with small amounts of Zr or Hf as substitutional atoms), the binary borides Mo₂B, MB and MB₂. Recently, it was [...] Read more.

The Molybdenum rich ternary alloys Mo-M-B (M = Zr, Hf) contain, next to the Mo solid solution (bcc Mo with small amounts of Zr or Hf as substitutional atoms), the binary borides Mo₂B, MB and MB₂. Recently, it was found that there is also ternary Mo₂MB₂, but the crystal structure and further properties are currently unknown. Density functional theory (DFT) calculations were used not only to predict the crystal structure of the Mo₂MB₂ phases, but also to estimate the isotropic and anisotropic elastic properties like bulk, shear and Young’s modulus, as well as the Vickers hardness of these new borides. Several known crystal structures that fulfill the criterion of the chemical composition were investigated, and the AlMn₂B₂ type structure seems to be the most stable crystal structure for Mo₂HfB₂ and Mo₂ZrB₂ as there are no signs of electronic or dynamic instability. Regarding the elastic properties, it was found that Mo₂HfB₂ shows higher elastic moduli and is less elastically anisotropic than Mo₂ZrB₂. Full article

(This article belongs to the Special Issue Intermetallic)

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9 pages, 4142 KiB

Open AccessArticle

Growth and Magnetism of Mn_xGe_1−x Heteroepitaxial Quantum Dots Grown on Si Wafer by Molecular Beam Epitaxy

by Maolong Yang, Liming Wang, Jie You, Lingyao Meng, Yichi Zhang, Bo Wang, Bin Wang and Huiyong Hu

Crystals 2020, 10(6), 534; https://doi.org/10.3390/cryst10060534 - 23 Jun 2020

Cited by 1 | Viewed by 2001

Abstract

Self-assembled MnGe quantum dots (QDs) were grown on Si (001) substrates using molecular beam epitaxy with different growth temperatures and Ge deposition thicknesses to explore the interaction among Mn doping, Ge deposition, the formation of intermetallics, and the ferromagnetism of QDs. With the [...] Read more.

Self-assembled MnGe quantum dots (QDs) were grown on Si (001) substrates using molecular beam epitaxy with different growth temperatures and Ge deposition thicknesses to explore the interaction among Mn doping, Ge deposition, the formation of intermetallics, and the ferromagnetism of QDs. With the introduction of Mn atoms, the QDs become large and the density significantly decreases due to the improvement in the surface migration ability of Ge atoms. The growth temperature is one of the most important factors deciding whether intermetallic phases form between Mn and Ge. We found that Mn atoms can segregate from the Ge matrix when the growth temperature exceeds 550 °C, and the strongest ferromagnetism of QDs occurs at a growth temperature of 450 °C. As the Ge deposition thickness increases, the morphology of QDs changes and the ferromagnetic properties decrease gradually. The results clearly indicate the morphological evolution of MnGe QDs and the formation conditions of intermetallics between Mn and Ge, such as Mn₅Ge₃ and Mn₁₁Ge₈. Full article

(This article belongs to the Special Issue Intermetallic)

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