Structural relationships, phase stability and bonding of compounds PdSnn (n = 2, 3, 4)

摘要

The Sn rich part of the binary system Pd-Sn has been investigated for possible temperature dimorphism. Well-shaped single crystals of PdSn_4, PdSn_3 and PdSn_2 were obtained under self-flux condition where the stannide is in equilibrium with the melt. The melt was subsequently removed by centrifugation at the synthesis temperature. The obtained products were characterised by X-ray diffraction, thermal analysis and 119Sn M?ssbauer spectroscopy. The crystal structures of the obtained products corresponded to the already assigned structure types PtSn_4, PdSn_3 and PdSn_2. The structures of PdSn_n (n = 2, 3, 4) represent stackings of similar building blocks and are closely related. Structural stability of compounds PdSnn (n = 2, 3, 4) with respect to different stacking possibilities of building blocks was investigated by ab initio calculations in the framework of density functional theory. It was found that differently stacked alternatives to the experimentally determined structure types are very close in energy. Nevertheless, experimentally no indications of polytype formation or even stacking disorder were obtained for PdSn_n (n = 2, 3, 4). The peritectic decompositions of PdSnn were investigated by differential scanning calorimetry. The obtained decomposition temperatures are 330, 370, 617 ± 3 ℃ for n = 4, 3, 2, respectively. The ~(119)Sn Mossbauer isomer shifts of compounds PdSn_n are relatively high (δ ≈ 2.4 mm s~(-1)) and decrease with increasing Pd content. This behaviour coincides with the trend in the number of occupied Sn 5s states as obtained from theoretical calculations.