The crystal structure of quaternary (Sn,Pb,Bi)Pt

The crystal structure of quaternary (Sn,Pb,Bi)Pt adopts the NiAs structure type with additional occupation of voids.


Chemical context
Platinum-based intermetallic compounds possess promising properties as electrocatalysts and provide necessary stability for the harsh application conditions in acidic electrolytes (Rö ssner & Armbrü ster, 2019). SnPt, PbPt and BiPt are interesting electrocatalysts for the oxidation of small organic molecules and have the NiAs type of crystal structure (Oftedal, 1928;Nowotny et al., 1946;Zhuravlev et al., 1962). So far, the existence of a substitutional solid solution between PtPb and PtBi was confirmed by powder X-ray diffraction, with the site occupancy deduced from the nominal composition (Zhuravlev et al., 1962), which also holds for all three binary end members. To obtain material for electrocatalytic investigations, the synthesis of single-phase (Sn,Pb,Bi)Pt was attempted. Large hexagonal crystals were found on the top of an otherwise microgranular ingot. Preliminary EDXS analysis indicated the presence of all four elements in the crystal. Further structural investigations besides the original structure reports for PtSn (Harris et al., 1968;Shelton et al., 1981;Durussel et al., 1994), PtPb (Zhuravlev et al. 1962;Sidorov et al., 2021) and PtBi (Zhuravlev & Stepanova, 1962a,b) provide no full structural characterization by means of single-crystal X-ray diffraction. Thus, structural data for binary, ternary or quaternary samples in the (Sn,Pb,Bi)Pt system are incomplete. To provide such data, one of the obtained crystals was studied by means of single-crystal X-ray diffraction.

Structural commentary
As a result of the very similar scattering power of three of the four atoms (Bi, Pb and Pt), the direct assignment of the atomic positions to the respective elements was not possible. Atoms were distributed based on crystal-chemical considerations as well as by achieving an agreement between the refined composition and the result of the EDXS analysis (Fig. 1). The 2a site was assigned to Pt in agreement with structural studies of binary endmembers. A mixed occupancy of Sn, Pb and Bi was assumed for the 2c position. The statistical distribution of these elements at the same atomic site is based on the full miscibility of the elements in the molten state and on the missing site preference in the only known binary phase Pb 0.7 Bi 0.3 (Mg type of crystal structure; Kurnakov & Ageeva, 1937). Additional electron density was detected on the 2c (6m2) and 4f (3m.) sites, for which two possible scenarios can be considered. Either those positions are occupied by the smaller Sn atoms as a result of the enlarged unit-cell volume of 84.84 Å 3 , which is 7.2% higher compared to 79.14 Å 3 for SnPt (Oftedal, 1928), or the presence of stacking faults. Neither can be proven here.
As a result of the potential partial occupation of 2c (6m2) and 4f (3m.) in the hexagonal lattice of the quaternary sample, we assign the crystal structure to the NiAs type. The refined composition of 7.5% at Sn, 27.0% at Pb, 15.5% at Bi and 50% at Pt is in broad agreement with the results of EDXS measurements (12.35% at Sn, 25.87% at Pb, 9.49% at Bi and 52.29% at Pt) considering the error of this method, which to our experience is up to 5% at for standardless quantifications of non-ideal samples, i.e. mirror-finished surfaces.

Synthesis and crystallization
Elements were weighed in an Ar-filled glove-box (O 2 and H 2 O content < 0.1 ppm) according to the nominal composition of 20.83% at Sn (99.999%, granules, ChemPUR), 20.83% at Pb (99.999%, granules, AlutervFKI), 8.33% at Bi (99.997%, granules, AlfaAesar) and 50.00% at Pt (99.95%, foil, Goodfellow), then sealed in an evacuated silica glass ampoule. The ampoule was placed into a furnace at 1473 K for 24 h, then cooled down from 1473 K to 873 K at a rate of 0.2 K min À1 . The temperature of 873 K was held for seven days and subsequently the ampoule was quenched in cold water. Single crystals with a hexagonal shape were selected from the top of an otherwise microgranular sample, which was composed of phases with the Cu 3 Au and NiAs type of crystal structure, based on powder X-ray diffraction data. As a result of the high X-ray absorption of the investigated material, hexagonalshaped specimens were too large for single crystal X-ray data collection. For this experiment, a relatively small piece was mechanically separated from a hexagonally shaped block. The composition of the investigated single crystal was determined by EDXS (Quantax, Bruker).

Refinement
Crystallographic data, data collection and structure refinement details are summarized in Table 1.
To decrease the number of parameters, the Pt site was constrained to full occupation at the 2a (3m.) site. Even though the standardless quantification by means of EDXS data is 52.3% at Pt, recent results of bulk samples from the quasi-ternary cut of the quaternary Sn-Pb-Bi-Pt system indicate a strict upper compositional limit of 50% at Pt (Rö ssner et al., 2023). An initial refinement was done for Pb and Bi, using EDXS values as a starting point, then the additional electron density was considered by adding Sn. After multiple cycles, it was decided that a compromise had to be made between excellent refinement results and compositions close to the ones from EDXS results. The final model is presented here.  Furthermore, it has to be noted that Sn3 was refined with isotropic displacement parameters, as the minor site occupancy (2.7%), does not justify to add additional parameters to enable a refinement with anisotropic displacement parameters. It has to be stressed that the ratio of 13 parameters for 123 independent reflections is already at the recommended upper limit (ratio parameters:reflections < 1:10). SHELXL (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg & Putz, 2018); software used to prepare material for publication: SHELXL (Sheldrick, 2015). Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.