A new representative of the cubic parkerites family: Synthesis, crystal and electronic structure of Pt3Bi2Se2

Journal of Alloys and Compounds 651 (2015) 193e199

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A new representative of the cubic parkerites family: Synthesis, crystal and electronic structure of Pt3Bi2Se2 Elena Yu. Zakharova a, Sergey M. Kazakov a, Alexey N. Kuznetsov a, b, * a b

Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1-3, GSP-1, 119991 Moscow, Russian Federation N.S. Kurnakov Institute of General and Inorganic Chemistry of Russian Academy of Sciences, Leninskii Pr. 31, 119991 Moscow, Russian Federation

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 July 2015 Received in revised form 13 August 2015 Accepted 17 August 2015 Available online 19 August 2015

A new mixed platinum-bismuth selenide, Pt3Bi2Se2, was synthesized by a high-temperature ampoule technique. Its crystal structure was determined by Rietveld method from X-ray powder diffraction data. Pt3Bi2Se2 crystallizes in the cubic system with I213 space group: a ¼ 8.49336(5) Å, Z ¼ 4, Rp ¼ 0.037, Rb ¼ 0.014. The compound represents a rare example of the cubic parkerite structure and is isotypic with Pd3Bi2S2. Its electronic structure was evaluated from quantum chemical calculations on the density functional theory level. Pt3Bi2Se2 is found to exhibit metallic properties and Pauli-like paramagnetic behavior. Chemical bonding was evaluated using the combination of Bader's QTAIM approach and the topological analysis of electron localizability indicator. According to that data, three dimensional framework of Pt3Bi2Se2 is largely based on covalent PteSe and polar covalent PteBi pairwise interactions. © 2015 Elsevier B.V. All rights reserved.

Keywords: Transition metal chalcogenides Parkerites Crystal structure Electronic band structure X-ray diffraction

1. Introduction In our research, we have been concentrating on investigating new compounds based on heterometallic bond systems between transition and main-group metals. Particularly those in which such metallic systems would be diluted by a non-metal, such as a chalcogen or a pnictogen, since this might lead to unusual structures and unconventional properties of such compounds. This approach, applied to the palladium (platinum) e heavier group 13e14 metal systems, previously allowed us to discover several new chalcogenides and arsenides based on well-established intermetallic motifs of the Cu3Au (Pd5InSe, Pd8In2Se2 [1], Pd17In4Se4 [2], Pt8In2As2 [3], Pd7-xSnTe2 [4]) or NiAs (Pd3PbTe2, Pd3SnTe2 [5]) structure types. Upon extending our search to the palladium (platinum) e heavy group 15 metal systems, we have discovered a new compound in the metal-rich region of the PteBieSe system, whose powder pattern bore striking similarities to the cubic parkerite or ‘halfantiperovskite’ Pd3Bi2S2 that was reported in 2006 by Weihrich et al. [6,7]. Indeed, further studies have confirmed that a new compound that we discovered was Pt3Bi2Se2, and it does represent

* Corresponding author. Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1-3, GSP-1, 119991 Moscow, Russian Federation. E-mail address: [email protected] (A.N. Kuznetsov). http://dx.doi.org/10.1016/j.jallcom.2015.08.133 0925-8388/© 2015 Elsevier B.V. All rights reserved.

a rare cubic variety of a parkerite-type structure. This has been a fairly surprising find for two reasons. Firstly, it appears that since the discovery of the first member of the class in the late thirties e Ni3Bi2S2, the parkerite mineral itself [8], and a structurally closely related shandite, Ni3Pb2S2 [9], ternary systems of the TM-M-Q type (TM e group 10 or group 9 metal, M e heavy main-group metal, Q e S or Se) have been studied rather extensively in a search for the 3:2:2 compositions, which resulted in the discovery and structural characterization of the whole families of parkerite- and shanditerelated compounds, mostly group 10 metal-based, i.e. nickel [10] or palladium [6,7,11] mixed sulphides and selenides, although there are also examples of cobalt- [10f,12] and rhodium-based [13] ones. The interest in those compounds was recently renewed with the discovery of superconductivity in several representatives of the class [14]. Yet platinum, despite being a group 10 metal along with nickel and palladium, was almost never featured in those compounds. To this day, there have been only two platinum-based shandites (and no parkerites) mentioned in the literature, Pt3Pb2S2 and Pt3Pb2Se2, with the structural data so far only available for the former and only as a part of a PhD Thesis [15]. And secondly, up to now, almost all shandites were found to adopt trigonal rhombohedral symmetry, while parkerites crystallize in the monoclinic system, and among many members of the family there has been only a single example of a cubic parkerite, Pd3Bi2S2 [6,7], thus making our compound all the more interesting.

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Here we report the synthesis, as well as crystal and electronic structure, of a second representative of cubic parkerite-like chalcogenides, Pt3Bi2Se2.

Table 1 Crystallographic parameters, data collection and refinement details for Pt3Bi2Se2.

2. Experimental 2.1. Synthetic and analytical procedures Platinum (powder, 99.8%), bismuth (powder, 99.999%), and elemental selenium (shot, 99.999%) were used for the synthesis. Stoichiometric mixtures of the elements (ca. 0.3e0.4 g in total per sample) were put into dried silica ampoules, sealed under vacuum (ca. 25 mTorr), and annealed at 750  C for 10 days, then cooled down, homogenized by a thorough grinding in an agate mortar, then annealed at 600  C for another 10 days. Phase purity of the products was confirmed by powder X-ray diffraction (XRD) using Stoe STADI-P automated diffractometer (CuKa1). Electrondispersive spectroscopic (EDS) analysis was performed using Jeol/ Nikon Neoscope JCM-6000 scanning microscope with built-in analytical module (accelerating voltage 15 kV). The composition of microcrystals was established by averaging the results obtained from three-point measurements on each of several microcrystals (acquisition time ~ 180 s per point).

Compound

Pt3Bi2Se2

Data collection Radiation type, source Data collection temperature, K Range in 2q, step size ( ) Space group Z Formula weight (g/mol) Unit cell parameter a (Å) V (Å3) Rp, Rwp RBragg GoF

Huber G670 X-ray, CuKa1 295 11.2e84.0, 0.02 I213 (No. 199) 4 4644.44 8.49336(5) 612.69(1) 0.037, 0.047 0.014 1.32

Table 2 Refined coordinates and isotropic displacement parameters for Pt3Bi2Se2. Atom

Site

x/a

y/b

z/c

Biso, Å2

Bi Pt Se

8a 12b 8a

0.01693(5) 0.29189(8) 0.2838(1)

0.01693(5) 0 0.2838(1)

0.01693(5) 1/4 0.2838(1)

0.9(3) 1.4(3) 1.0(3)

2.2. Crystal structure determination X-ray powder diffraction patterns were recorded using a Huber G670 automated powder diffractometer equipped with an imaging plate detector (CuKa1-radiation, Ge(111)-monochromator, transmission geometry). The crystal structure of Pt3Bi2Se2 was refined using the Rietveld method as implemented in the TOPAS package [16]. The structure of Pd3Bi2S2 [6,7] was used as a starting model for the refinement. Rietveld refinement was performed using the fundamental parameter approach for the peak shape description. Preferred orientation was corrected using a spherical harmonics approach as implemented in the TOPAS package. 3. Computational details Band structure calculations were performed on a densityfunctional theory (DFT) level utilizing the all-electron full-potential linearized augmented plane wave method (FP-LAPW) as implemented in the ELK code [17]. The Brillouin zone sampling was performed using 176 irreducible k-points. The PBESol exchangecorrelation functional [18] of the GGA-type was used in the calculations. The muffin-tin sphere radii for the respective atoms are (Bohr): 2.79 (Bi), 2.44 (Pt), 2.10 (Se). The maximum moduli for the reciprocal vectors kmax were chosen so that RMTkmax ¼ 10.0. The convergence criteria were: absolute change in total energy <105 Hartree, RMS change in Kohn-Sham potential <106 Hartree. The convergence of the total energy with respect to the k-point sets was checked. Atomic charges were analyzed according to Bader's QTAIM approach [19]. The electron localizability indicator (ELI-D) was calculated according to [20] using DGrid package [21]. The calculations were performed using the Intel Core-i7-based laboratory cluster and the MSU Lomonosov supercomputer [22]. Visualization of the electron localization indicators and their topological analysis was performed using ParaView package [23].

Table 1e2, and selected interatomic distances e in Table 3. Final Rietveld refinement plot is shown in Fig. 1. According to the Rietveld refinement data, the formula of the new compound is found to be Pt3Bi2Se2, which is in a very good agreement with the Pt3.01(1)Bi2.00(1)Se1.99(2) composition obtained from the EDS data. The compound crystallizes in a cubic crystal system and is indeed isotypic with previously discovered Pd3Bi2S2 [6,7]. It formally belongs to the corderoite (Hg3S2Cl2) structure type [24], which is a cubic variant of the parkerite structure, so we, after the authors of [6,7], will use the term ‘cubic parkerite’ in relation to our compound, as it is more common and informative. The cubic unit cell of Pt3Bi2Se2 is shown in Fig. 2. As in the Pd3Bi2S2 structure, Bi in Pt3Bi2Se2 is coordinated by three Pt atoms at the relatively short distances of 2.792 Å, and there are three more Pt atoms at the longer distance of 3.065 Å. Whether the latter distance should be considered bonding is not clear from purely crystallographic data. For the comparison, BiePd distances in Pd3Bi2S2 were 2.7892(9) Å and 3.054(1) Å, and the latter atoms were included in the Bi coordination sphere [6]. Platinum atoms have two bismuth atoms at 2.792 Å each, and another two at 3.065 Å, and also have two selenium neighbors at 2.429 Å (the respective PdeS distances in Pd3Bi2S2 are 2.340(3) Å). All other interatomic distances are clearly non-bonding (see Table 3). There are no convenient for the visualization regular coordination polyhedra in this three-dimensional framework structure, so in order to display its regular arrangement we have arbitrarily chosen the distorted tetrahedra formed by two selenium and two bismuth atoms around platinum (the arbitrary

Table 3 Selected interatomic distances for Pt3Bi2Se2 (bold highlights bonding distances). Atoms

Distance, Å

4. Results and discussion

Bi e Pt

4.1. Crystal structure description

Pt e Se Bi e Se

Crystallographic data, atomic coordinates and isotropic thermal displacement parameters for the new compound are given in

Bi e Bi Pt e Pt

2.7923(4) 3.0649(7) 2.429(1) 3.642(1) 3.760(1) 3.9695(5) 3.2834(8)

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Fig. 1. Observed, calculated and difference Rietveld profile for Pt3Bi2Se2.

low-temperature cubic K2Sn2O3 polytype. Applying this line of reasoning to the structure of our Pt3Bi2Se2, we can describe this cubic phase both as ‘half-antiperovskite’ cubic K2Sn2O3 antitype where K positions are occupied by Bi, Sn e by Se, and O e by Pt atoms, and in terms of the relation to the CsCl-type structure, where Bi and Se form a distorted CsCl-type superstructure, with Pt occupying half of the Bi4Se2 sites.

4.2. Electronic structure and bonding

Fig. 2. Unit cell of Pt3Bi2Se2. Platinum atoms are shown as grey, bismuth as blue, selenium as yellow spheres. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

nature of such tetrahedron being the part where Pt is residing in the middle of its edge rather than in its centre, see Fig. 3). The SeePteSe angle is 176.76(4)o, and BiePteBi is 108.89(3)o. Another way to describe the structures of shandites and parkerites is through their relation to the CsCl structure type, as shown by Range et al. [10g], or through type-antitype relationship with perovskite structure, and the key to structural variations is the transition metal ordering. This has been meticulously described in the works of Weihrich et al. [7,11] that have solidified the term ‘halfantiperovskites ‘ in regard to the compounds in question and highlighted a type-antitype relationship between the shandite structure and potassium oxostannate(II), K2Sn2O3, similar to perovskite-antiperovskite. Similarly, it has been shown [6,7] that the cubic perovskite Pd3Bi2S2 can be described as an antitype of

Fig. 3. Polyhedral representation of the Pt3Bi2Se2 structure motif.

Electronic structure of Pt3Bi2Se2 was evaluated from the DFT calculations data, both with and without spineorbit coupling (SOC) effects and spin-polarization. The respective total densities of states (TDOS) near the Fermi level are shown in Fig. 4. As seen from the figure, the effects of SOC are only quantitative and do not alter general TDOS picture, according to which the compound is a metallic conductor (as was found for Pd3Bi2S2 [6,7]). Based on the similarity of a and b spin DOS in spin-polarized calculations, we can assume that the spin-ordering effects are rather unlikely for this compound, and it most probably exhibits a Pauli-like paramagnetic behavior, typical for non-magnetic metals. From the l-resolved atomic projections of DOS (see Fig. 5) we can clearly see that the main contributions near the Fermi level arise from Pt 5d- and Se 4p-electrons, and to a slightly lesser extent from Bi 6p-electrons. All these states reside in the same energy range between 0.25 and 0.15 Hartree and appear to be mixed, while well-localized Bi 6s lone pairs and Se 4s lone pairs are below 0.4 Hartree and clearly do not participate in bonding. However, the fact that different states occupy the same energy range cannot be taken as a proof of a bond between respective elements, other bonding indicators must be utilized for a proper analysis. In order to gain more insight into the chemical bonding in Pt3Bi2Se2, we have employed direct-space charge density partitioning and topological analysis of electron localizability indicator (ELI-D). Atomic charges, calculated according to the Bader's QTAIM approach [19], are: Pt 0.44, Bi þ0.83, Se 0.17. These figures indicate that there is a partial charge transfer from bismuth towards platinum and selenium atoms, however, it is far too small to consider significant ionic contribution to the bonding. Thus, charge density indicates predominantly covalent nature of interatomic interactions, the ones with the bismuth participation showing the degree of polarity. This picture can be further amended using topological analysis of ELI-D [20], which is ideologically close to the well-known electron localization function (ELF). ELI-D topology is analyzed in the same way as ELF one, i.e. upon decreasing the localization parameter, maxima corresponding to atomic shells, lone pairs, or bonds can be observed. At high values of localizability parameter (Y), only atomic shells are evident in the ELI-D topology for Pt3Bi2Se2. Below Y~1.45, selenium lone pairs start to appear (labeled A in Fig. 6), and upon further decrease of Y bismuth lone

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Fig. 4. Total density of states (TDOS) near the Fermi level for Pt3Bi2Se2: no spineorbit coupling effects (left) and spineorbit coupling effects and spin-polarization included (right). The Fermi level is at zero.

pairs also become visible (labeled B in Fig. 7). Typically for d-metals, especially heavier ones, ELI-D (or ELF) maxima corresponding to bonds with their participation appear at relatively low values of localizability parameter, when the picture becomes cluttered by sizeable atomic shells. Nevertheless, ELI-D maxima corresponding to the disynaptic basins of pairwise covalent PteBi (labeled C in Fig. 8) and PteSe (labeled D in Fig. 8) interactions can be observed below Y ¼ 0.996. Topological analysis of ELI-D isosurfaces also shows that those maxima correspond only to the shorter BiePt distances (2.792 Å), and not to the longer (3.065 Å) ones (Fig. 9), which means that the nature of bonding between bismuth and three closer platinum atoms is different from that between it and three others, and the latter ones probably should not be included in bismuth coordination sphere in Pt3Bi2Se2. Summarizing the bonding pattern obtained from the two approaches, we can state

that the 3D framework in Pt3Bi2Se2 is predominantly based on the covalent PteSe and polar covalent BiePt pairwise interactions. Comparing our results of electronic structure calculations and bonding analysis with those published for Pd3Bi2S2 [7], we see that they are mostly in good agreement. Total and projected densities of states correlate well between the two compounds, despite the fact that the calculations in Ref. [7] were done on a slightly lower level of theory (not fully relativistic, LDA description of exchangecorrelation potential). Qualitative bonding description in the momentum space, derived in Ref. [7] from covalent bond energy (ECOV [25]), that is a slightly amended crystal orbital overlap population (COOP [26]) and crystal orbital Hamiltonian population (COHP [27]), agrees with our picture regarding the bonding PdeS and PdeBi interactions. In addition, ECOV in Ref. [7] shows bonding PdePd interactions of the same magnitude as PdeS and BiePd

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Fig. 5. Projected densities of states (PDOS) for Pt, Bi, and Se in Pt3Bi2Se2. The Fermi level is at zero.

197

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Fig. 9. A fragment of the ELI-D cross-section in the plane of the PdeBi bonds. Contour lines correspond to the ELI-D isosurfaces between Y ¼ 1.30 and Y ¼ 0.90 with the step of 0.05.

Fig. 6. ELI-D isosurface (Y ¼ 1.400) for Pt3Bi2Se2.

Fig. 7. ELI-D isosurface (Y ¼ 1.001) for Pt3Bi2Se2.

Fig. 8. ELI-D isosurface (Y ¼ 0.990) for Pt3Bi2Se2.

interactions, which is rather surprising since the typical bond distances between palladium atoms are ~2.6e2.8 [1,2] Å, with 2.75 Å in the palladium metal itself, which is considered unusually long among transition metals [28]. In Pd3Bi2S2, however, PdePd distance exceeds 3.05 Å. At the same time, ECOV features equally strong antibonding PdePd interactions, so the resulting type of PdePd bonding is ambiguous and is not commented upon in Ref. [7]. For our compound, direct-space analysis shows no indication of PtePt bonding, and since the PtePt distances (~3.06 Å) in Pt3Bi2Se2 are similar to the PdePd distances in Pd3Bi2S2, strong covalent bonding between the transition metal atoms is unlikely to be expected in the latter compound either. If we compare the electronic structure and bonding of the cubic parkerites to the monoclinic Pd3Bi2Se2, the one that is chemically closest to them, we see a similar picture. According to [11], the bands directly below the Fermi level are dominated by the palladium 4d electrons, with smaller contribution from bismuth 6p and selenium 4p electrons. Bonding analysis, based on the ECOV approach, shows covalent PdeSe (both bonding and antibonding) and BieSe (mostly bonding) interactions [11], but the magnitude of the latter is almost 10 times lower, which indicates rather weak bonding. The biggest difference, pointed out in Ref. [11] between the cubic and monoclinic palladium-based parkerites, is probably more anisotropic character of the electronic structure of the latter, due to the change from the 3D PdeS network in Pd3Bi2S2 to the quasi 2D PdeSe network in Pd3Bi2Se2. Looking at the broader picture, we can also draw some comparisons between the electronic structure of our cubic parkerite and other half-antiperovskites. While the amount of data on parkerites is rather limited, there have been numerous recent DFT studies on various shandites, mostly for the 3d-metals [12a,13,29e31], with the exception of Rh3Sn2S2 [13], with the bonding analysis for many shandites performed using various approaches, including ELF and QTAIM. While the qualitative DOS picture in those compounds remains similar and rather characteristic for this type of compounds (i.e. major contribution from the dstates of a transition metal right below the Fermi level, smaller contributions from p-states of main-group elements, significant mixing of the states), lower symmetry and 2D nature of the networks provide shandites with more varying transport and magnetic properties arising from anisotropic structures, particularly for the quaternary shandites of the Co3InxSn2-xS2 type where the ordering of main-group metals also plays a crucial part [31]. Chemical bonding in the shandites in question is described in terms of multi-centered bonds between d-metal and p-metal, prevalently covalent bonds between d-metal and sulphur, and prevalently ionic between p-metal and sulphur. This correlates well with our analysis

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of the bonding in Pt3Bi2Se2. Our compound is more isotropic in terms of both structure and electronegativity of the constituent elements, and it is only reasonable for it to feature a more homogeneous bond network. Also, the lack of observed multi-centered heterometallic bonds in our case is in good agreement with our findings that platinum in metal-rich compounds tends to exhibit more localized bonding as compared to lighter group 10 metals, such as palladium [3]. 5. Conclusions Using the high-temperature ampoule technique, we have synthesized a new mixed platinum-bismuth selenide, Pt3Bi2Se2, and determined its crystal structure from powder XRD data. The compound represents a rare example of cubic parkerite-like structure and is found to be isotypic with previously discovered Pd3Bi2S2. Electronic structure of the new selenide was evaluated based on the DFT calculations that predict Pt3Bi2Se2 to be a 3D metallic conductor and Pauli-like paramagnet. According to the bonding analysis based on the ELI-D topology and Bader's QTAIM approach, the Pt3Bi2Se2 framework predominantly consists of the PteSe and PteBi covalent interactions, the latter showing a degree of polarity. Considering the similarities that Pt3Bi2Se2 shows to Pd3Bi2S2, it is feasible that more platinum-containing chalcogenides of the shandite or parkerite structure type can be discovered in the future. Acknowledgments This work was partially supported by Russian Foundation for Basic Research (grants 14-03-31802 and 15-03-06459a). The use of the resources of MSU Supercomputer Center is kindly acknowledged. The use of scanning microscopy equipment was supported by the Moscow State University Development Programme. References [1] E.Yu. Zakharova, S.M. Kazakov, A.A. Isaeva, A.M. Abakumov, G. Van Tendeloo, A.N. Kuznetsov, J. Alloys Compd. 589 (2014) 48e55. [2] E.Yu. Zakharova, A.V. Churakov, Th. Doert, A.N. Kuznetsov, Eur. J. Inorg. Chem. (2013) 6164e6169. [3] E.Yu. Zakharova, N.A. Andreeva, S.M. Kazakov, A.N. Kuznetsov, J. Alloys Compd. 621 (2015) 307e313. [4] S.V. Savilov, A.N. Kuznetsov, B.A. Popovkin, V.N. Khrustalev, P. Simon, J. Getzschmann, Th. Doert, M. Ruck, Z. Anorg. Allg. Chem. 631 (2005) 293e301. [5] E.Yu. Zakharova, A.A. Isaeva, A.N. Kuznetsov, A.V. Olenev, Russ. Chem. Bull. 60 (2011) 440e445. [6] R. Weihrich, I. Anusca, Z. Anorg. Allg. Chem. 632 (2006) 335e342. [7] R. Weihrich, S. Matar, V. Eyert, F. Rau, M. Zabel, M. Andratschke, I. Anusca, Th. Bernert, Prog. Solid State Chem. 35 (2007) 309e327. [8] D.L. Scholtz, Trans. Geol. Soc. S. Afr. 39 (1937) 81e210. [9] a) J.W. DuPreez, Ann. Univ. Stellenbosch 22A (1944) 94e104; b) P. Ramdohr, Sitzungsber. Dt. Akad. Wissensch 6 (1949) 1e29.

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