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Crystals 2018, 8(4), 169; doi:10.3390/cryst8040169

Article
Crystal Structure, Spectroscopic Investigations, and Physical Properties of the Ternary Intermetallic REPt2Al3 (RE = Y, Dy–Tm) and RE2Pt3Al4 Representatives (RE = Tm, Lu)
Fabian Eustermann 1, Simon Gausebeck 1, Carsten Dosche 2, Mareike Haensch 2Orcid, Gunther Wittstock 2Orcid and Oliver Janka 1,2,*Orcid
1
Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstrasse 30, 48149 Münster, Germany
2
Institut für Chemie, Carl von Ossietzky Universität Oldenburg, 26111 Oldenburg, Germany
*
Correspondence: ocjanka@uni-muenster.de; Tel.: +49-251-83-36074
Received: 1 March 2018 / Accepted: 10 April 2018 / Published: 16 April 2018

Abstract

:
The REPt2Al3 compounds of the late rare-earth metals (RE = Y, Dy–Tm) were found to crystallize isostructural. Single-crystal X-ray investigations of YPt2Al3 revealed an orthorhombic unit cell (a = 1080.73(6), b = 1871.96(9), c = 413.04(2) pm, wR2 = 0.0780, 942 F2 values, 46 variables) with space group Cmmm (oC48; q2pji2hedb). A comparison with the Pearson database indicated that YPt2Al3 forms a new structure type, in which the Pt and Al atoms form a [Pt2Al3]δ polyanion and the Y atoms reside in the cavities within the framework. Via a group-subgroup scheme, the relationship between the PrNi2Al3-type structure and the new YPt2Al3-type structure was illustrated. The compounds with RE = Dy–Tm were characterized by powder X-ray diffraction experiments. While YPt2Al3 is a Pauli-paramagnet, the other REPt2Al3 (RE = Dy–Tm) compounds exhibit paramagnetic behavior, which is in line with the rare-earth atoms being in the trivalent oxidation state. DyPt2Al3 and TmPt2Al3 exhibit ferromagnetic ordering at TC = 10.8(1) and 4.7(1) K and HoPt2Al3 antiferromagnetic ordering at TN = 5.5(1) K, respectively. Attempts to synthesize the isostructural lutetium compound resulted in the formation of Lu2Pt3Al4 (Ce2Ir3Sb4-type, Pnma, a = 1343.4(2), b = 416.41(8), c = 1141.1(2) pm), which could also be realized with thulium. The structure was refined from single-crystal data (wR2 = 0.0940, 1605 F2 values, 56 variables). Again, a polyanion with bonding Pt–Al interactions was found, and the two distinct Lu atoms were residing in the cavities of the [Pt3Al4]δ framework. X-ray photoelectron spectroscopy (XPS) measurements were conducted to examine the electron transfer from the rare-earth atoms onto the polyanionic framework.
Keywords:
intermetallics; crystal structure; group-subgroup; magnetic properties; XPS

1. Introduction

In the field of intermetallic compounds [1,2], some structure types are found with an impressive number of entries listed in the Pearson database [3]. Amongst them are the binary Laves phases of the MgCu2-type (Fd 3 ¯ m) [4] and MgZn2-type (P63/mmc) [5] structures (together with more than 5500 entries), the cubic Cu3Au-type (Pm 3 ¯ m, >1950 entries) structures [6], and the hexagonal CaCu5-type (P6/mmm, >1650 entries) structures [7]. For ternary intermetallic compounds, the tetragonal body-centered ThCr2Si2-type (I4/mmm, >3250 entries) [8], the orthorhombic TiNiSi-type (Pnma, >1550 entries), and the hexagonal ZrNiAl-type (P 6 ¯ 2m, >1450 entries) [9] representatives show a broad variety of compounds with numerous, different elemental combinations. The structures and physical properties of the equiatomic RETX (RE = rare-earth element, T = transition metal, X = element of group 12–15) representatives have been recently summarized in a series of review articles [10,11,12,13].
Derived from the binary CaCu5-type structure, two prototypic ternary representatives with different chemical compositions have been reported: the CeCo3B2- [14] and the PrNi2Al3-type [15] structures. From a crystal chemical point of view, YNi2Al3 is also worth mentioning [16], because this compound can be considered to be an i3-superstructure of the PrNi2Al3-type structure. Recently, an i7-superstructure of PrNi2Al3 has also been reported, which was also found for ErPd2Al3 [17]. Our interests in the compounds of the REPt2Al3 series originate from the fact that only CePt2Al3 (PrNi2Al3-type) has been reported previously [18]. Therefore, we synthesized and characterized the missing members of the REPt2Al3 series with the late, small rare-earth elements. From a basic research point of view, investigations of the magnetic ground state of the open f-shell rare-earth atoms are also of great interest.

2. Experimental

2.1. Synthesis

The starting materials for the synthesis of the REPt2Al3 and RE2Pt3Al4 samples were pieces of the sublimed rare-earth elements (Y, Dy–Tm, and Lu from Smart Elements), platinum sheets (Agosi), and aluminum turnings (Koch Chemicals), all with stated purities better than 99.9%. For the REPt2Al3 compounds (RE = Y, Dy–Tm), the elements were weighed in the ideal 1:2:3 atomic ratio and arc-melted [19] in a water-cooled copper hearth under 800 mbar of argon pressure. The argon gas was purified with a titanium sponge (873 K), molecular sieves, and silica gel. Re-melting of the obtained buttons from each site several times enhanced the homogeneity. The as-cast buttons of the yttrium compound were crushed, and the fragments were sealed in quartz ampoules, placed in the water-cooled sample chamber of a high-frequency furnace (Typ TIG 5/300, Hüttinger Elektronik, Freiburg, Germany) [20], and heated until a softening of the piece was observed. The power was subsequently reduced by 10%, and the sample was kept at this temperature for 120 min before being cooled to room temperature. The other samples were annealed in muffle furnaces. They were heated to 1223 K and then kept at this temperature for 14 days, followed by slow cooling until they reached 573 K. Afterwards, the furnace was switched off. These different annealing procedures led to X-ray pure samples suitable for physical properties measurements. For the RE2Pt3Al4 compounds (RE = Tm, Lu), the elements were weighed in the ideal 2:3:4 atomic ratio and arc-melted as described above. Again, an annealing step in a high-frequency furnace was subsequently conducted. The specimens are stable in air over weeks and show metallic luster; the ground samples are grey.

2.2. X-ray Image Plate Data and Data Collections

The polycrystalline samples were characterized at room temperature by powder X-ray diffraction on a Guinier camera (equipped with an image plate system, Fujifilm, Nakanuma, Japan, BAS-1800,) using Cu Kα1 radiation and α-quartz (a = 491.30, c = 540.46 pm, Riedel-de-Haën, Seelze, Germany) as an internal standard. The lattice parameters (Table 1) were obtained from a least-squares fit. Proper indexing of the diffraction lines was ensured by an intensity calculation [21].
Irregularly shaped crystal fragments of the YPt2Al3 and Lu2Pt3Al4 compounds were obtained from the annealed crushed buttons. The crystals were glued to quartz fibers using beeswax, and their quality was checked by Laue photographs on a Buerger camera (white molybdenum radiation, image plate technique, Fujifilm, Nakanuma, Japan, BAS-1800) for intensity data collection. The datasets were collected on a Stoe StadiVari four-circle diffractometer (Mo-Kα radiation (λ = 71.073 pm); µ-source; oscillation mode; hybrid-pixel-sensor, Dectris Pilatus 100 K [22]) with an open Eulerian cradle setup. Numerical absorption correction along with scaling was applied to the datasets. All relevant crystallographic data, deposition, and details of the data collection and evaluation are listed in Table 2, Table 3, Table 4, Table 5, Table 6, Table 7 and Table 8. Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-Mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request_for_deposited_data.html) by quoting the depository numbers CSD-434174 (YPt2Al3) and CSD-434175 (Lu2Pt3Al4).

2.3. Energy Dispersive X-ray Spectroscopy (EDX) Data

The crystals measured on the diffractometer were analyzed semi-quantitatively using a Zeiss EVO MA10 scanning electron microscope with YF3, TmF3, LuF3, Pt, and Al2O3 as standards. No impurity elements heavier than sodium (the detection limit of the instrument) were observed. The experimentally determined element ratios (YPt2Al3: 18 ± 2 at.% Y: 29 ± 2 at.% Pt: 53 ± 2 at.% Al; and Lu2Pt3Al4: 20 ± 2 at.% Y: 36 ± 2 at.% Pt: 44 ± 2 at.% Al) were in close agreement with the ideal compositions (16.7:33.3:50 and 22.2:33.3:44.5), respectively. The deviations resulted from the irregular shape of the crystal surfaces (conchoidal fracture). Additionally, polycrystalline pieces from the annealed arc-melted buttons were embedded in a methylmethacrylate matrix and polished with diamond and SiO2 emulsions of different particle sizes. During the first attempts to synthesize TmPt2Al3 and LuPt2Al3, phase segregation was observed; the secondary phases had the compositions Tm2Pt3Al4 and Lu2Pt3Al4.

2.4. Magnetic Properties Measurements

Fragments of the annealed buttons of the X-ray pure REPt2Al3 phases were attached to the sample holder rod of a vibrating sample magnetometer (VSM) unit using Kapton foil for measuring the magnetization M(T, H) in a Quantum Design physical property measurement system (PPMS). The samples were investigated in the temperature range of 2.5–300 K with external magnetic fields up to 80 kOe. The magnetic data are summarized in Table 9.

2.5. X-ray Photoelectron Spectroscopy (XPS)

XPS was performed using an ESCALAB 250 Xi instrument (Thermo Fisher, East Grinsted, UK) with mono-chromatized Al Kα ( = 1486.6 eV) radiation. All samples were cleaned by Ar+ sputtering (MAGCIS ion gun, 36 keV) for 60 s to remove adventitious carbon. High-resolution spectra were measured with pass energies of 10 eV (Pt 4f, Al 2s, Al 2p, and C 1s) and 20 eV (Y 3d and Pr 3d). Peak deconvolution was performed using a Gaussian-Lorentzian peak shape by the software Avantage (Thermo Fisher). All spectra were referenced to remaining adventitious carbon at 284.8 eV. Because of the overlap of the Pt 4f and Al 2p signals, Al 2s was used for Al quantification. The obtained data are summarized in Table 10.

3. Results and Discussion

During attempts to synthesize aluminum intermetallics with the composition REPt2Al3, well-resolved X-ray powder patterns for the small rare-earth elements RE = Y, Dy–Tm were observed. For the thulium compound, additional reflections showed up in the unannealed sample, which were initially interpreted as impurities. Subsequently, single crystals from the yttrium sample were isolated and structurally investigated (vide infra). The large and early rare-earth elements (RE = La–Nd, Sm, Gd, and Tb) do not form the same structure type. Investigations on the structures formed by these elements are still ongoing. Attempts to synthesize LuPt2Al3 also yielded a diffraction pattern different from the slightly larger rare-earth elements Dy–Tm. As cast specimen, TmPt2Al3 and LuPt2Al3 were subsequently investigated by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDX). The impurity phase in TmPt2Al3 and the main phase in nominal LuPt2Al3 were found to be Tm2Pt3Al4 and Lu2Pt3Al4, respectively. Finally, samples with these compositions were prepared, and single crystals from Lu2Pt3Al4 were isolated and investigated.

3.1. Structure Refinements

A careful analysis of the obtained intensity dataset of YPt2Al3 revealed an orthorhombic C-centered lattice. The centrosymmetric group Cmmm was found to be correct during structure refinement. A systematic check of the Pearson database [3], using Pearson code oC48 and Wyckoff sequence q2pji2hedb, gave no matches; hence, YPt2Al3 must be considered a new structure type. The starting atomic parameters were obtained using SuperFlip [23], implemented in Jana2006 [24,25]. The structure was refined on F2 with anisotropic displacement parameters for all atoms. As a check for the correct composition and site assignment, the occupancy parameters were refined in a separate series of least-squares cycles. All sites were fully occupied within three standard deviations. No significant residual peaks were evident in the final difference Fourier syntheses. At the end, the positional parameters were transformed to the setting required for the group-subgroup scheme discussed below. Figure 1 depicts the X-ray powder diffraction pattern of YPt2Al3 along with the calculated pattern obtained using the positional information from the single-crystal structure refinement.
Lu2Pt3Al4 was also found to crystallize in the orthorhombic crystal system with space group Pnma. A comparison with the Pearson database [3], using Pearson code oP36 and Wyckoff sequence c9, indicated isotypism with Ce2Ir3Sb4 [26,27]. The structure was refined on F2 with anisotropic displacement parameters for all atoms. As a check for the correct composition and site assignment, the occupancy parameters were refined in a separate series of least-squares cycles. All sites were fully occupied within three standard deviations. No significant residual peaks were evident in the final difference Fourier syntheses. In the powder X-ray diffraction experiments, trace amounts of TmPtAl or LuPtAl (TiNiSi-type) were evident. Thermal treatment was not able to remove these impurities. The details of the structure refinement, final positional parameters, and interatomic distances are listed in Table 2, Table 3, Table 4, Table 5, Table 6, Table 7 and Table 8.

3.2. The YPt2Al3-Type Structure: Crystal Chemistry and Group-Subgroup Relations

The isostructural aluminum compounds of the REPt2Al3 series (RE = Y, Dy–Tm) crystallize in the orthorhombic crystal system, space group Cmmm, Pearson code oC48 and Wyckoff sequence q2pji2hedb. The lattice parameters (Figure 2) and unit cell volumes (Table 1) decrease from the dysprosium to the thulium compound, as expected, from the lanthanide contraction. The lattice parameters of the yttrium compound are in the same range, explainable by the similar ionic radii (Y3+: 90 pm; Dy3+: 91 pm; Ho3+: 90 pm [28]).
As YPt2Al3 was investigated by single-crystal X-ray diffraction experiments, its crystal structure will be used for the structural discussion. A view of the crystal structure along the crystallographic c axis is depicted in Figure 3. The crystal structure features a polyanionic [Pt2Al3]δ network and shows full Pt/Al ordering. The heteroatomic Pt–Al distance range from 253 to 261 pm indicates substantial Pt–Al bonding, because these distances are in the range of the sum of the covalent radii for Pt+Al of 129 + 125 = 254 pm [29]. The polyanionic networks of YPtAl (TiNiSi-type) [30] and Y4Pt9Al24 (Y4Pt9Al24-type) [31] show similar distances of 257–269 and 246–274 pm, respectively. Additionally, homoatomic Al–Al distances ranging from 274 to 280 pm, and Pt–Pt distances of 301 pm can be found. The latter distances are slightly longer compared to what is found in elemental Pt (Cu-type, 284 pm) [32], while the aluminum distances are in line with elemental Al (Cu-type, 286 pm) [33]. Three crystallographically distinct Y3+ cations can be found in the cavities of the polyanion. They exhibit 18-fold coordination environments in the shape of six-fold-capped hexagonal prisms (Figure 4). The hexagonal prisms have slightly different compositions of Y1@[Al12+Pt6], Y2@[Al8Pt4+Al4Pt2], and Y3@[Al8Pt4+Al4Pt2]. The Y–Pt distances range from 301 to 316 pm; the Y–Al distances are 329 pm. While the former distances are in line with YPtAl, the latter distances are significantly longer (Y–Pt: 304–320 pm; Y–Al: 287–305 pm) [30].
A view of the unit cell along the c axis readily reminds us of the ternary CaCu5-type derivatives PrNi2Al3 [15], YNi2Al3 [16], DyNi4Si [34], CeCo3B2 [14], and the recently found i7 superstructure of PrNi2Al3 [17]. Recoloring in intermetallics is found quite frequently, often accompanied by distortions and puckering within the respective structures [35]. These structural effects between different structure types can be investigated by so-called group-subgroup relations. The structures of PrNi2Al3 and YPt2Al3 are related by such a group-subgroup scheme, which is presented in the Bärnighausen formalism [36,37,38,39] in Figure 5. In the first step, an isomorphic symmetry reduction of index 4 takes place, which causes a doubling of the a and b axis, along with a splitting of the Pr (1a to 1a and 3f), Ni (2c to 2c and 6l), and Al (3g to 6k and 6m) sites. In the second step a translationengleiche transition of index 3 takes place, reducing the hexagonal symmetry from space group P6/mmm to orthorhombic Cmmm. Again, a splitting of the crystallographic position occurs along with the introduction of additional degrees of freedom regarding the crystallographic positions. This enables a distortion of the polyanion and a recoloring of the crystallographic sites. The Y1 atoms finally occupy the 2d rather than the 2a site as suggested by the group-subgroup scheme. Hence, they are shifted by 1/2 z compared to the original position. The same shift is also observed in YNi2Al3 [16,35] and i7-PrNi2Al3 [17]. Refinement as orthorhombic trilling, as suggested by the translationengleiche symmetry reduction of index 3, is not necessary because the orthorhombic crystal system was found directly by the indexing routine.

3.3. Crystal Chemistry of Tm2Pt3Al4 and Lu2Pt3Al4

Tm2Pt3Al4 and Lu2Pt3Al4 crystallize in the orthorhombic crystal system with space group Pnma (oP36, c9) in the Ce2Ir3Sb4-type structure [26,27]. In the following paragraph, Lu2Pt3Al4 will be used for the structure description. As in the REPt2Al3 series, the platinum and aluminum atoms form a network. Figure 6 depicts the extended unit cell along [010], and the polyanionic [Pt3Al4]δ network and the two different lutetium sites are highlighted. The heteroatomic Pt–Al distances span a larger range (246–269 pm) compared to YPt2Al3; however, Pt–Al bonding is still present. In contrast to YPt2Al3, only additional Al–Al bonds can be found ranging from 278 to 300 pm. In the polyanion, no Pt–Pt bonds below 400 pm are found. The Al atoms form corrugated layers consisting of rectangles and hexagons in the boat conformation (Figure 7, top) that are capped by the Pt atoms (Figure 7, bottom).
The lutetium cations occupy two distinct crystallographic sites and are again found in the cavities of the polyanion. Lu1 is surrounded by 16 atoms in a four-fold-capped hexagonal prismatic environment (Lu1@[Al6Pt6+Al4]; Figure 8, top), while Lu2 has a three-fold-capped pentagonal prismatic coordination sphere (Lu2@[Al6Pt4+Al2Pt]; Figure 8, bottom). The Lu–Pt distances range from 299 to 310 pm, and the Lu–Al distances range from 327 to 347 pm. The Lu–Pt distances are in line with LuPtAl; the Lu–Al contacts are significantly longer (Lu–Pt: 302–327 pm; Lu–Al: 284–301 pm) [30].

3.4. Magnetic Properties

Magnetic susceptibility data has been obtained for the X-ray pure REPt2Al3 samples with RE = Y, Dy–Tm. The basic magnetic parameters that have been derived from these measurements are listed in Table 9. The temperature dependence of the magnetic susceptibility of the yttrium compound is depicted in Figure 9. YPt2Al3 is a Pauli-paramagnetic material with a room temperature susceptibility of χ = 1.85(1) × 10–4 emu mol–1. The weak upturn at lower temperature arises from small amounts of paramagnetic impurities. The present data clearly proves the absence of local moments on all constituent atoms. Thus, the magnetic properties of the remaining phases arise solely from the rare-earth elements.
The magnetic properties of DyPt2Al3, HoPt2Al3, ErPt2Al3, and TmPt2Al3 have been depicted in Figure 10, Figure 11, Figure 12 and Figure 13. The top panels always depict the susceptibility and inverse susceptibility data (χ and χ–1). The effective magnetic moments have been obtained from fitting the χ–1 data using the Curie–Weiss law between 50 and 300 K. They were calculated from the Curie constant according to μ eff = 3 k B C N A [40,41]. All rare-earth atoms are in the trivalent oxidation state; the effective magnetic moments compare well within the calculated moments, as stated in Table 9. The calculated moments are tabulated [40,41] or can be calculated according to μ calc = g J ( J + 1 ) with g =   1 + J ( J + 1 ) + S ( S + 1 ) L ( L 1 ) 2 J ( J + 1 ) [40,41].
Because a positive Weiss constant of θP is observed for the antiferromagnetically ordered compounds, the ordering phenomena could be a so-called Type-A antiferromagnetic ground state. In this ordered state, the intra-plane coupling is ferromagnetic while inter-plane coupling is antiferromagnetic [42]. From the zero-field-cooled/field-cooled (ZFC/FC) measurements depicted in the middle panels, it is evident that DyPt2Al3 and TmPt2Al3 exhibit ferromagnetic ordering at Curie temperatures of TC = 10.8(1) and 4.7(1) K due to the plateau-like susceptibility at low temperatures. ErPt2Al3 exhibits no magnetic ordering down to 2.5 K, while HoPt2Al3 finally orders antiferromagnetically at TN = 5.5(1) K, characterized by decreasing susceptibility below the Néel temperature. The Curie temperatures were obtained from the derivatives dχ/dT of the field-cooled curves (depicted in red) by determination of the temperature at the minimum in the derivative curve. The bottom panels finally display the magnetization isotherms measured at 3, 10, and 50 K. The 3 K isotherms of DyPt2Al3 and TmPt2Al3 show a fast increase at low fields, in line with the ferromagnetic ground state. The 3 K isotherm of HoPt2Al3 displays a slightly delayed increase, suggesting a spin-reorientation, in line with a weak antiferromagnetic ground state. The 3 K isotherm of DyPt2Al3 displays small ‘wiggles’, suggesting trace impurities, which are hardly noticeable in the ZFC/FC measurements. In the 3 K isotherm of HoPt2Al3, a small bifurcation is visible, also suggesting trace impurities, visible around 3 K in the ZFC/FC measurements. The isotherms at 50 K are all linear, in line with paramagnetic materials. The saturation magnetizations determined at 3 K and 80 kOe are all below the calculated values according to gJ × J (Table 9). The extracted values are, in all cases, lower than the expected moments, suggesting that the applied external field is not strong enough to achieve full parallel spin ordering.

3.5. X-ray Photoelectron Spectroscopy

The reported compounds were described by rare-earth cations located in the cavities of a polyanion. Hence, the rare-earth atoms transfer electron density to the framework. This is in line with the effective magnetic moments of the rare-earth cations, proving them to be formally in a trivalent oxidation state. When looking at the electronegativities χ of the constituting elements of the REPt2Al3 series, it is evident that platinum is by far the most electronegative element. According to the Pauling scale, the values are as follows: χ(Y) = 1.22, χ(Dy) = 1.22, χ(Ho) = 1.23, χ(Er) = 1.24, χ(Tm) = 1.25, χ(Pt) = 2.28, and χ(Al) = 1.61 [30]. Because all reported compounds are of a metallic nature, a distinct ionic platinide character as found in A2Pt (A = K [43], Rb [43], Cs [43,44]) is highly unlikely, especially when considering the three-dimensional framework with strong covalent bonding character formed by Pt and Al. Therefore, XPS measurements were performed to investigate exemplarily the platinide character of YPt2Al3 along with the reference substances YPt5Al2 (anti-ZrNi2Al5-type [45]), YPtAl (TiNiSi-type [31]), and elemental Pt.
The obtained binding energies are listed in Table 10. Figure 14 depicts an exemplary fitted spectrum of YPt2Al3. As observed for Ba3Pt4Al4 (Eb(Pt 4f7/2) = 70.9 eV) [46], the binding energies of YPt2Al3 (Eb(Pt 4f7/2) = 70.4 eV), YPt5Al2 (Eb(Pt 4f7/2) = 70.6 eV), and YPtAl (Eb(Pt 4f7/2) = 70.2 eV) are all shifted towards lower binding energies in comparison with elemental Pt (Eb(Pt 4f7/2) = 71.2 eV). This can be explained by a higher electron density at the Pt atoms, in line with an electron transfer from the less electronegative Y and Al atoms. The existing literature [46] shows shifts of the Pt 4f7/2 signal towards higher binding energies for the binary phases PtAl and PtAl2 (PtAl: 71.6, PtAl2: 72.1 eV), which can be explained by the bond formation between Pt and Al. In the ternary compounds, the additional electron transfer from the rare-earth atoms causes the lower binding energies and the ‘platinide’ character. While YPtAl and YPt2Al3 exhibit extensive Pt–Al bonding within the polyanion, only few heteroatomic Pt–Al bonds are observed in Pt-rich YPt5Al2. Consequently, the spectra of YPt5Al2 show the smallest shift in comparison with elemental Pt. In YPtAl, an equal ratio of Pt and Al can be found in contrast with YPt2Al3. In the latter compound, additional homoatomic bonding takes place; therefore, YPt2Al3 shows a smaller shift in the Pt 4f7/2 binding energies than YPtAl. As expected, Y is acting as electron donor, and therefore, the Y 3d5/2 signal is shifted by approximately 1 eV to higher binding energies (c.f. Table 10). However, all samples show a minor Y 3d5/2 component, that appears around 155.5 eV, in line with possible contaminations by traces of elemental yttrium.

4. Conclusions

Attempts to synthesize the CaCu5-type related compounds REPt2Al3 with the late rare-earth elements Dy–Tm and Y led to the discovery of a new structure type, which was refined from single-crystal data obtained for YPt2Al3. The structure crystallizes in the orthorhombic space group Cmmm and can be derived from CaCu5 by distortion and recoloring of the framework. Attempts to synthesize LuPt2Al3 led to the discovery of Lu2Pt3Al4 (Ce2Ir3Sb4-type), which was also refined from single-crystal data. The REPt2Al3 compounds could be obtained in phase pure form for property investigations. While YPt2Al3 is Pauli-paramagnetic, DyPt2Al3 to TmPt2Al3, in contrast, show paramagnetism in line with formal RE3+ cations, along with magnetic ordering for RE = Dy, Ho, and Tm at low temperatures. Via XPS investigations, the binding energies of the constituent elements were investigated and compared with the electronegativities. In comparison with reference substances, the expected charge transfer onto the Pt atoms within the polyanionic [Pt2Al3]δ network could be proven.

Acknowledgments

We thank Dipl.-Ing. Ute Ch. Rodewald for its collection of the single-crystal diffractometer data. The XPS facility has been co-funded by the Deutsche Forschungsgemeinschaft (INST 184/144-1 FUGG).

Author Contributions

Fabian Eustermann and Simon Gausebeck performed the synthesis and the powder diffraction experiments; Fabian Eustermann, Simon Gausebeck and Oliver Janka solved and refined the single crystal structures; Carsten Dosche and Mareike Haensch measured and analyzed the XPS spectra; Oliver Janka measured and analyzed the magnetic data. Carsten Dosche, Gunther Wittstock and Oliver Janka wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Experimental (top) and calculated (bottom) Guinier powder pattern (CuKα1 radiation) of YPt2Al3.
Figure 1. Experimental (top) and calculated (bottom) Guinier powder pattern (CuKα1 radiation) of YPt2Al3.
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Figure 2. Plot of the unit cell parameters of the REPt2Al3 phases as a function of the rare-earth element.
Figure 2. Plot of the unit cell parameters of the REPt2Al3 phases as a function of the rare-earth element.
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Figure 3. The crystal structure of YPt2Al3. Yttrium, platinum, and aluminum atoms are drawn as green/blue, black-filled, and open circles, respectively. The polyanionic [Pt2Al3]δ network is highlighted.
Figure 3. The crystal structure of YPt2Al3. Yttrium, platinum, and aluminum atoms are drawn as green/blue, black-filled, and open circles, respectively. The polyanionic [Pt2Al3]δ network is highlighted.
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Figure 4. Coordination polyhedra surrounding the three crystallographically independent yttrium sites in YPt2Al3. Yttrium, platinum, and aluminum atoms are drawn as green/blue, black-filled, and open circles, respectively. The local site symmetries are given.
Figure 4. Coordination polyhedra surrounding the three crystallographically independent yttrium sites in YPt2Al3. Yttrium, platinum, and aluminum atoms are drawn as green/blue, black-filled, and open circles, respectively. The local site symmetries are given.
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Figure 5. Group-subgroup scheme in the Bärnighausen formalism [36,37,38,39] for the structures of PrNi2Al3 and YPt2Al3. The index for the isomorphic (i) and translationengleiche (t) symmetry reduction, the unit cell transformation, and the evolution of the atomic parameters are given.
Figure 5. Group-subgroup scheme in the Bärnighausen formalism [36,37,38,39] for the structures of PrNi2Al3 and YPt2Al3. The index for the isomorphic (i) and translationengleiche (t) symmetry reduction, the unit cell transformation, and the evolution of the atomic parameters are given.
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Figure 6. Extended crystal structure of Lu2Pt3Al4 along [010]. Lutetium, platinum, and aluminum atoms are drawn as green/blue, black-filled, and open circles, respectively. The polyanionic [Pt3Al4]δ network and the two different coordination environments for the lutetium atoms are highlighted.
Figure 6. Extended crystal structure of Lu2Pt3Al4 along [010]. Lutetium, platinum, and aluminum atoms are drawn as green/blue, black-filled, and open circles, respectively. The polyanionic [Pt3Al4]δ network and the two different coordination environments for the lutetium atoms are highlighted.
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Figure 7. The Al arrangement in the crystal structure of Lu2Pt3Al4 (top). The Pt atoms capping the layers are depicted in the bottom image. Platinum and aluminum atoms are drawn as black-filled and open circles, respectively. The Pt–Al bonds in the polyanionic [Pt3Al4]δ network are highlighted.
Figure 7. The Al arrangement in the crystal structure of Lu2Pt3Al4 (top). The Pt atoms capping the layers are depicted in the bottom image. Platinum and aluminum atoms are drawn as black-filled and open circles, respectively. The Pt–Al bonds in the polyanionic [Pt3Al4]δ network are highlighted.
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Figure 8. Coordination polyhedra surrounding the two crystallographically independent lutetium sites in Lu2Pt3Al4. Lutetium, platinum, and aluminum atoms are drawn as green/blue, black-filled, and open circles, respectively. The local site symmetries are given.
Figure 8. Coordination polyhedra surrounding the two crystallographically independent lutetium sites in Lu2Pt3Al4. Lutetium, platinum, and aluminum atoms are drawn as green/blue, black-filled, and open circles, respectively. The local site symmetries are given.
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Figure 9. Temperature dependence of the magnetic susceptibility (data) of YPt2Al3 measured at 10 kOe.
Figure 9. Temperature dependence of the magnetic susceptibility (data) of YPt2Al3 measured at 10 kOe.
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Figure 10. Magnetic properties of DyPt2Al3: (top) temperature dependence of the magnetic susceptibility χ and its inverse χ–1 measured at 10 kOe; (middle) zero-field-cooled/field-cooled (ZFC/FC) data (100 Oe) and the dχ/dT derivative (red curve) of the FC curve; and (bottom) magnetization isotherms recorded at 3, 10, and 50 K.
Figure 10. Magnetic properties of DyPt2Al3: (top) temperature dependence of the magnetic susceptibility χ and its inverse χ–1 measured at 10 kOe; (middle) zero-field-cooled/field-cooled (ZFC/FC) data (100 Oe) and the dχ/dT derivative (red curve) of the FC curve; and (bottom) magnetization isotherms recorded at 3, 10, and 50 K.
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Figure 11. Magnetic properties of HoPt2Al3: (top) temperature dependence of the magnetic susceptibility χ and its inverse χ–1 measured at 10 kOe; (middle) zero-field-cooled/field-cooled (ZFC/FC) data (100 Oe); and (bottom) magnetization isotherms recorded at 3, 10, and 50 K.
Figure 11. Magnetic properties of HoPt2Al3: (top) temperature dependence of the magnetic susceptibility χ and its inverse χ–1 measured at 10 kOe; (middle) zero-field-cooled/field-cooled (ZFC/FC) data (100 Oe); and (bottom) magnetization isotherms recorded at 3, 10, and 50 K.
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Figure 12. Magnetic properties of ErPt2Al3: (top) temperature dependence of the magnetic susceptibility χ and its inverse χ–1 measured at 10 kOe; (middle) zero-field-cooled/field-cooled (ZFC/FC) data (100 Oe); and (bottom) magnetization isotherms recorded at 3, 10, and 50 K.
Figure 12. Magnetic properties of ErPt2Al3: (top) temperature dependence of the magnetic susceptibility χ and its inverse χ–1 measured at 10 kOe; (middle) zero-field-cooled/field-cooled (ZFC/FC) data (100 Oe); and (bottom) magnetization isotherms recorded at 3, 10, and 50 K.
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Figure 13. Magnetic properties of TmPt2Al3: (top) temperature dependence of the magnetic susceptibility χ and its inverse χ–1 measured at 10 kOe; (middle) zero-field-cooled/field-cooled (ZFC/FC) data (100 Oe) and the dχ/dT derivative (red curve) of the FC curve; and (bottom) magnetization isotherms recorded at 3, 10, and 50 K.
Figure 13. Magnetic properties of TmPt2Al3: (top) temperature dependence of the magnetic susceptibility χ and its inverse χ–1 measured at 10 kOe; (middle) zero-field-cooled/field-cooled (ZFC/FC) data (100 Oe) and the dχ/dT derivative (red curve) of the FC curve; and (bottom) magnetization isotherms recorded at 3, 10, and 50 K.
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Figure 14. Fitted X-ray photoemission spectrum of Pt 4f in YPt2Al3. The experimental data is shown as black squares, the Pt 4f components are depicted in green, the Al 2p lines in blue, and the envelope function in red. The background is depicted as a dashed line.
Figure 14. Fitted X-ray photoemission spectrum of Pt 4f in YPt2Al3. The experimental data is shown as black squares, the Pt 4f components are depicted in green, the Al 2p lines in blue, and the envelope function in red. The background is depicted as a dashed line.
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Table 1. Lattice parameters of the orthorhombic REPt2Al3 series (YPt2Al3-type, rare-earth (RE) = Y, Dy–Tm), space group Cmmm, and RE2Pt3Al4 series (Ce2Ir3Sb4-type, RE = Y, Dy–Tm), space group Pnma.
Table 1. Lattice parameters of the orthorhombic REPt2Al3 series (YPt2Al3-type, rare-earth (RE) = Y, Dy–Tm), space group Cmmm, and RE2Pt3Al4 series (Ce2Ir3Sb4-type, RE = Y, Dy–Tm), space group Pnma.
Compounda (pm)b (pm)c (pm)V (nm³)
YPt2Al31080.73(6)1871.96(9)413.04(2)0.8356
DyPt2Al31081.3(1)1872.7(2)413.93(5)0.8382
HoPt2Al31079.26(4)1869.46(6)413.55(2)0.8344
ErPt2Al31077.31(6)1866.0(1)413.14(4)0.8305
TmPt2Al31075.38(9)1862.6(1)412.87(4)0.8270
Tm2Pt3Al41349.9(3)418.22(8)1143.7(2)0.6429
Lu2Pt3Al41343.4(2)416.41(8)1141.1(2)0.6383
Table 2. Crystallographic data and structure refinement for YPt2Al3, space group Cmmm, Z = 8, own type and Lu2Pt3Al4, space group Pnma, Z = 4, Ce2Ir3Sb4-type.
Table 2. Crystallographic data and structure refinement for YPt2Al3, space group Cmmm, Z = 8, own type and Lu2Pt3Al4, space group Pnma, Z = 4, Ce2Ir3Sb4-type.
CompoundYPt2Al3Lu2Pt3Al4
Molar mass, g mol–1560.01043.1
Density calc., g cm–38.9310.91
Crystal size, µm 25 × 40 × 5530 × 30 × 40
Detector distance, mm4040
Exposure time, s2550
Integr. param. A, B, EMS6.2; −5.2; 0.0175.0; −4.1; 0.012
Range in hkl±16; ±28, ±6±21; ±6, ±18
θmin, θmax, deg 2.2–32.92.3–35.5
Linear absorption coeff., mm–181.297.0
No. of reflections11,71421,601
Rint/Rσ0.1124/0.01780.1411/0.1152
No. of independent reflections 9421605
Reflections used [I ≥ 3σ(I)]795679
F(000), e18721712
R1/wR2 for I ≥ 3σ(I)0.0341/0.07700.0415/0.0798
R1/wR2 for all data0.0422/0.07800.1095/0.0940
Data/parameters 942/461605/56
Goodness-of-fit on F22.221.23
Extinction coefficient161(17)73(6)
Diff. Fourier residues/e Å–3−4.15/3.97−4.98/4.51
Table 3. Atom positions and equivalent isotropic displacement parameters (pm2) for YPt2Al3. Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.
Table 3. Atom positions and equivalent isotropic displacement parameters (pm2) for YPt2Al3. Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.
AtomWyckoffxyzUeq
Position
Y12d001/2151(7)
Y24e1/41/40137(4)
Y32b1/200137(6)
Pt14h0.27855(6)01/2120(2)
Pt28q0.13928(4)0.13927(3)1/2120(1)
Pt34i00.33333(4)0136(2)
Al14j00.2483(3)1/2128(14)
Al28q0.3729(4)0.1244(2)1/2138(10)
Al38p0.2244(4)0.0748(2)0160(11)
Al44i00.1494(4)0157(16)
Table 4. Atom positions and equivalent isotropic displacement parameters (pm2) for Lu2Pt3Al4. Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. y = 1/4 all 4c.
Table 4. Atom positions and equivalent isotropic displacement parameters (pm2) for Lu2Pt3Al4. Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. y = 1/4 all 4c.
AtomxzUeq
Lu10.01840(10)0.71349(12)199(3)
Lu20.29143(10)0.57858(14)218(3)
Pt10.13365(9)0.24522(11)196(3)
Pt20.38024(9)0.06876(11)201(3)
Pt30.62220(9)0.58482(11)189(3)
Al10.0017(7)0.0827(9)210(2)
Al20.0714(8)0.4553(8)180(20)
Al30.3017(7)0.8651(9)190(30)
Al40.3174(7)0.2828(9)170(20)
Table 5. Anisotropic displacement parameters (pm2) for YPt2Al3. Coefficients Uij of the anisotropic displacement factor tensor of the atoms are defined by: −2π2[(ha*)2U11+ ... + 2hka*b*U12]. U13 = U23 = 0.
Table 5. Anisotropic displacement parameters (pm2) for YPt2Al3. Coefficients Uij of the anisotropic displacement factor tensor of the atoms are defined by: −2π2[(ha*)2U11+ ... + 2hka*b*U12]. U13 = U23 = 0.
AtomU11U22U33U12
Y1144(10)139(11)169(14)0
Y2137(7)137(7)136(9)−1(6)
Y3135(10)140(10)136(12)0
Pt1126(3)122(3)112(3)0
Pt2117(2)131(2)112(3)7(1)
Pt3147(3)153(3)107(4)0
Al1160(2)120(20)110(30)0
Al2138(16)145(17)130(20)−18(14)
Al3210(20)156(18)110(20)40(15)
Al4110(20)220(30)140(30)0
Table 6. Anisotropic displacement parameters (pm2) for Lu2Pt3Al4. Coefficients Uij of the anisotropic displacement factor tensor of the atoms are defined by: −2π2[(ha*)2U11 + ... + 2hka*b*U12]. U13 = U23 = 0.
Table 6. Anisotropic displacement parameters (pm2) for Lu2Pt3Al4. Coefficients Uij of the anisotropic displacement factor tensor of the atoms are defined by: −2π2[(ha*)2U11 + ... + 2hka*b*U12]. U13 = U23 = 0.
AtomU11U22U33U12
Lu1194(5)209(6)194(5)–4(4)
Lu2234(6)228(6)191(5)–5(5)
Pt1192(5)206(6)190(5)–14(4)
Pt2215(5)203(5)183(5)–15(4)
Pt3179(5)204(5)184(4)–9(4)
Al1200(40)220(40)210(40)10(4)
Al2280(50)130(40)120(30)30(3)
Al3120(40)210(50)240(50)0
Al4200(40)130(40)170(40)20(3)
Table 7. Interatomic distances (pm) for YPt2Al3. All distances of the first coordination spheres are listed. All standard uncertainties were less than 0.2 pm.
Table 7. Interatomic distances (pm) for YPt2Al3. All distances of the first coordination spheres are listed. All standard uncertainties were less than 0.2 pm.
Y1:2Pt1300.7Pt2:1Al1253.3Al2:1Pt2253.7
4Pt2300.7 1Al2253.7 1Pt1253.7
4Al4347.3 2Al4256.0 2Pt3260.1
8Al3347.6 2Al3256.0 1Al2274.4
1Pt1300.7 1Al1274.8
Y2:2Pt3311.7 1Y1300.7 2Al3277.3
4Pt2315.8 1Pt2300.7 2Y2339.6
2Al3328.8 1Y2315.8 2Y3339.9
2Al4329.0
4Al2339.6Pt3:4Al2260.1Al3:2Pt2256.0
4Al1339.7 2Al1260.5 2Pt1256.0
2Y2311.7 2Al2277.3
Y3:2Pt3311.7 1Y3311.7 1Al4279.6
4Pt2315.8 2Al3343.6 1Al3279.7
2Al3328.8 1Al4343.9 1Y3328.8
2Al4329.0 1Y2328.8
4Al2339.6Al1:2Pt2253.3 1Pt3343.6
4Al1339.7 2Pt3260.5 2Y1347.6
2Al2274.8
Pt1:2Al2253.9 2Al4277.1Al4:4Pt2256.0
4Al3256.0 4Y2339.7 2Al1277.1
2Pt2300.7 2Al3279.6
1Y1300.7 2Y2329.0
2Y3315.8 1Pt3343.9
2Y1347.6
Table 8. Interatomic distances (pm) for Lu2Pt3Al4. All distances of the first coordination spheres are listed. All standard uncertainties were less than 0.2 pm.
Table 8. Interatomic distances (pm) for Lu2Pt3Al4. All distances of the first coordination spheres are listed. All standard uncertainties were less than 0.2 pm.
Lu1:2Pt3298.8Pt2:2Al2253.1Al2:2Pt2253.1
2Pt1302.0 1Al4254.9 1Pt3253.6
2Pt2310.4 1Al2258.1 1Pt2258.1
1Al4326.8 1Al1258.1 2Al4287.6
1Al2327.1 2Lu2298.0 1Lu2302.7
1Al3336.8 2Lu1310.4 2Lu2307.6
1Al1338.9 1Lu1327.1
2Al4343.4Pt3:1Al1250.3
2Al1344.4 1Al2253.5Al3:1Pt1250.2
2Al3346.8 1Al3256.3 1Pt3256.3
2Al4266.3 2Pt1266.0
Lu2:1Pt1268.8 2Lu2294.9 2Al3280.5
2Pt3294.9 2Lu1298.8 1Al1291.0
2Pt2298.0 2Lu2312.7
1Al2302.7Al1:1Pt3250.3 1Lu1336.8
1Al4304.4 1Pt2258.1 2Lu1346.8
2Al2307.6 2Pt1269.1
2Al3312.7 1Al4278.3Al4:1Pt1247.6
2Al1312.9 1Al3291.0 1Pt2254.9
2Lu2312.9 2Pt3263.3
Pt1:1Al4247.6 1Lu1338.9 2Al1278.3
1Al3250.2 2Lu1344.4 2Al2287.6
2Al3266.0 1Lu2304.4
1Lu2268.8 2Lu1326.8
2Al1269.1 1Lu1343.4
2Lu1302.0
Table 9. Magnetic properties of the YPt2Al3-type compounds. TN, Néel temperature; TC, Curie temperature; μeff, effective magnetic moment; μcalc, calculated magnetic moment; θp, paramagnetic Curie temperature; μsat, saturation moment; and saturation according to gJ × J. The experimental saturation magnetizations were obtained at 3 K and 80 kOe.
Table 9. Magnetic properties of the YPt2Al3-type compounds. TN, Néel temperature; TC, Curie temperature; μeff, effective magnetic moment; μcalc, calculated magnetic moment; θp, paramagnetic Curie temperature; μsat, saturation moment; and saturation according to gJ × J. The experimental saturation magnetizations were obtained at 3 K and 80 kOe.
TN (K)TC (K)μeff (μB)μcalc (μB)θp (K)μsat (μB)gJ × J (μB)
YPt2Al3Pauli-paramagnetic χ(300 K) = 1.85(1) × 10–4 emu mol–1
DyPt2Al310.8(1)10.67(1)10.65+1.0(1)5.54(1)10
HoPt2Al35.5(1)10.59(1)10.61+2.0(1)7.23(1)10
ErPt2Al39.77(1)9.58+4.0(1)6.26(1)9
TmPt2Al34.7(1)7.69(1)7.56+12.8(1)4.25(1)7
Table 10. Fitted binding energies (in eV) determined by XPS of YPt2Al3, YPt5Al2, YPtAl, PrPtAl, and Pt and data from the literature. The determined uncertainty of binding energies in this work is ±0.1 eV.
Table 10. Fitted binding energies (in eV) determined by XPS of YPt2Al3, YPt5Al2, YPtAl, PrPtAl, and Pt and data from the literature. The determined uncertainty of binding energies in this work is ±0.1 eV.
CompoundPt 4f7/2Al 2sY 3d5/2Lit.
YPt2Al370.6117.2156.9*
YPt5Al270.9117.8157.0*
YPtAl70.4116.7156.6*
PrPtAl70.7***
Pt71.4*
Pt71.2[46]
Ba3Pt4Al470.9[46]
PtAl71.6[46]
PtAl272.1[46]
* This work. ** Signal invisible due to overlap with Pr 3d.

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