Redetermination of MoPt3Si4 from single-crystal data

Stephane Knittel; Michel Francois; Jean Steinmetz; Michel Vilasi

doi:10.1107/S1600536810054425

. 2011 Jan 22;67(Pt 2):i20. doi: 10.1107/S1600536810054425

Redetermination of MoPt₃Si₄ from single-crystal data

Stephane Knittel ^a, Michel Francois ^a, Jean Steinmetz ^a, Michel Vilasi ^a,^*

PMCID: PMC3051478 PMID: 21522816

Abstract

The crystal structure of molybdenum triplatinum tetrasilicide, MoPt₃Si₄, determined previously from powder diffraction data [Joubert et al. (2010 ▶). J. Solid State Chem. 183, 173–179], has been redetermined using a single crystal synthesized from the elements by high-frequency melting. The redetermination provides more precise geometrical data and also anisotropic displacement parameters. The crystal structure can be considered to be derived from the PtSi structure type with an ordered substitution of Pt by Mo atoms, but leading to a very distorted Si network compared to the parent structure. Mo and Pt exhibit different coordination polyhedra. These are based on bicapped-square antiprisms, but with two additional vertices in cis positions for Mo, whereas they are in trans positions for Pt (as in PtSi). The coordination polyhedra for three of the Si atoms can be considered as highly deformed square antiprisms (as in PtSi), while the fourth Si atom has a bicapped trigonal–prismatic coordination geometry.

Related literature

For general background to molybdenum silicides, see: Littner (2003 ▶); Benarchid et al. (2009 ▶); Bernard et al. (2010 ▶); Cabouro et al. (2007 ▶, 2008 ▶); Fitzer (1955 ▶); Knittel et al. (2010 ▶). For the structure determination of the title compound from standard X-ray powder diffraction data, see: Joubert et al. (2010 ▶). For the PAP correction program, see: Pouchou & Pichoir (1984 ▶).

Experimental

Crystal data

MoPt₃Si₄
M _r = 793.57
Orthorhombic,
a = 5.5121 (2) Å
b = 3.4951 (1) Å
c = 24.3078 (7) Å
V = 468.30 (3) Å³
Z = 4
Mo Kα radiation
μ = 92.80 mm⁻¹
T = 297 K
0.12 × 0.03 × 0.03 mm

Data collection

Bruker APEXII QUAZAR CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2004 ▶) T _min = 0.031, T _max = 0.174
9779 measured reflections
1060 independent reflections
955 reflections with I > 2σ(I)
R _int = 0.040

Refinement

R[F ² > 2σ(F ²)] = 0.029
wR(F ²) = 0.067
S = 1.41
1060 reflections
50 parameters
Δρ_max = 2.16 e Å⁻³
Δρ_min = −3.81 e Å⁻³

Data collection: APEX2 (Bruker, 2004 ▶); cell refinement: SAINT (Bruker, 2004 ▶); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: DIAMOND (Brandenburg, 1999 ▶) and ORTEP-3 (Farrugia, 1997 ▶); software used to prepare material for publication: SHELXL97.

Supplementary Material

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536810054425/fi2101sup1.cif

e-67-00i20-sup1.cif^{(25.9KB, cif)}

Structure factors: contains datablocks I. DOI: 10.1107/S1600536810054425/fi2101Isup2.hkl

e-67-00i20-Isup2.hkl^{(51.5KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Selected bond lengths (Å).

Pt1—Si3ⁱ	2.406 (3)
Pt1—Pt1ⁱⁱ	2.9119 (6)
Pt2—Si1ⁱⁱⁱ	2.387 (3)
Pt2—Mo1^iv	2.9331 (10)
Pt3—Si3	2.405 (2)
Pt3—Pt2	2.8874 (5)
Mo1—Si4	2.535 (2)
Mo1—Pt2ⁱⁱⁱ	2.9331 (10)
Si1—Pt2^iv	2.387 (3)
Si1—Si2	2.711 (3)
Si2—Pt3	2.466 (3)
Si2—Si1	2.711 (3)
Si3—Pt1^v	2.590 (2)
Si3—Si3^vi	2.807 (5)
Si4—Pt2^v	2.407 (2)
Si4—Si4^vii	3.012 (5)

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Symmetry codes: (i) Inline graphic ; (ii) ; (iii) ; (iv) ; (v) ; (vi) ; (vii) .

Acknowledgments

We thank Dr Holger Ott (Bruker AXS GmbH, Karlsruhe, Germany) for the data collection and the ’Service Commun de Microanalyse’ (Faculté des Sciences et Techniques, Vandoeuvre les Nancy, France).

supplementary crystallographic information

Comment

Interest for studying the Mo—MP—Si (MP = Pt, Ru) system follows from the attractive properties of MoSi₂ regarding high-temperature oxidation. But early in the past, the first studies performed by Fitzer (1955) mentioned the poor behaviour of this material under oxidizing atmosphere at moderate temperatures (300–600°C) due to a catastrophic degradation, the so-called "pest phenomenon". Consequently, one of the challenges for application of MoSi₂ is to control the pest oxidation by adding alloying elements (Ru, Pt, B, Al, Ge, Y, Ti, Zr, Ta) (Littner, 2003, Benarchid et al. 2009) and/or by controlling the microstructure. Recently, the optimization of the microstructure led to fully densified materials showing dramatically improved oxidation performance of MoSi₂ (Cabouro et al., 2007, Cabouro et al., 2008, Knittel et al., 2010), Bernard et al., 2010. In the course of our studies focused on the evaluation of the effect of elemental substitutions, we studied the isothermal section of the ternary phase diagram Mo—Pt—Si at 1423 K. Two new phases of composition MoPt₂Si₃ and MoPt₃Si₄ were identified (Littner, 2003), and MoPt₃Si₄ was indexed in the orthorhombic system: a = 5.5096 Å, b = 3.493 Å, c = 24.294 Å. The structure of MoPt₃Si₄ was recently published by Joubert et al. (2010), from powder X-ray diffraction data. It turned out that the MoPt₃Si₄ structure can be derived from PtSi by an ordered substitution of Pt by Mo. The atomic arrangement along the c axis leads to a fourfold superstructure, with c(MoPt₃Si₄) = 4 × c(PtSi). Our analysis based on single-crystal data confirms the previous results but yields more accurate atomic positions, approximately by one order of magnitude. Additionally, this also allows determination of anisotropic displacement parameters (Fig. 1). Fig. 2 shows the coordination polyhedra. The coordination number for each d metal is 10, for Si it is 8. Mo and Pt exhibit different coordination polyhedra. In both cases, these are based on bicapped-square antiprisms, but with two additional vertices in cis-positions for Mo whereas they are in trans-positions for Pt (as in PtSi). The coordination polyhedra for Si1, Si2 and Si3 atoms can be considered as highly deformed square antiprisms (as in PtSi), while Si4 has a bicapped trigonal prismatic coordination geometry. Shortest and longest interatomic distances for each coordination polyedra are reported in Table 1. The shortest distances found for Mo—Si and Pt—Si are 2.535 (2) Å and of 2.387 (3) Å respectively. These may be compared to the values given by Joubert et al. (2010), namely 2.463 Å and 2.355 Å.

Figure 3 also emphasizes that the corrugated ribbons formed by the silicon sub-network and expanded along the c axis are greatly distorted compared to the PtSi parent structure.

Experimental

Metal powders with nominal purities > 99.9 (Pt sponge 270 mesh - Engelhard - Clal, Si and Mo 325 mesh: Cerac) were mixed in different atomic ratios corresponding to alloys belonging to the MoSi₂—Mo₅Si₃—PtSi domain. An ingot was prepared by high frequency melting, and stabilized in a thermodynamic equilibrium for 100 h at 1150°C under argon. Single crystals of Pt₃MoSi₄ were directly isolated from the crushed ingot. A part of the ingot was embedded in an epoxy resin, polished and microanalyzed by an electron probe (SX 50 CAMECA, - PAP correction program (Pouchou & Pichoir, 1984). The EPMA composition corresponds, within the accuracy of the measurement, to that obtained by the structural determination.