The ternary compounds Pd13In5.25Sb3.75 and PdIn1.26Sb0.74:: Crystal structure and electronic structure calculations

The new ternary compound Pd13In5.25Sb3.75 was found. Its crystal structure was determined using a CCD diffractometer at room temperature. Evaluations and refinements finally yielded a C-centered monoclinic structure (space group, C2/c; Pearson symbol, mC88, Z= 4) with a = 15.189(2) Angstrom, b = 8.799(1) Angstrom, c = 13.602(2) Angstrom, and beta= 123.83(1). For the entire data set of 3706 independent reflections residual values are R = 0.0461 and R-w = 0.0789. The structure was found to be isotypic to Pd13Pb9 with In and Sb on the Pb sites. The existence of a further ternary compound, which was already described as Pd3In4Sb2, could be confirmed. Its composition range was determined by EPNIA to be PdIn1.2-1.3Sb0.8-0.7. It does not melt congruently and we were not able to find suitable single crystals. However, we were able to prepare the pure ternary compound in order to perform X-ray powder diffraction using a Guinier image plate technique. The entire diffraction spectrum was refined by full profile Rietveld method using the program Fullprof. The alpha-PdSn2 structure type (space group, 14(1)/acd; Pearson symbol, t148, Z= 16), proposed for this compound, was confirmed and the lattice parameters are a = 6.4350(1) Angstrom and c = 24.3638(3) Angstrom. The residual values were R-P = 5.34 and R-wp = 6.70. The tetragonal PdSn2 structure type is a mixed variant of the CaF2 type and the CuAl2 type structure. Also in this ternary compound we assumed a random contribution of In and Sb over the 16e and 16f positions. The electronic structures of both compounds were investigated by extended Huckel calculations. Crystal orbital overlap populations show extended bonding interactions between the main group elements. The bonding interactions of the main group elements are almost optimized at the experimentally observed In/Sb ratio of the ternary compound. The In/Sb ratio in Pd13In5.25Sb3.75 can thus be rationalized on the basis of the electronic structure. (C) 2002 Elsevier Science (USA).