Speaker
Description
Binary group IV-VI chalcogenides, like SnTe, GeTe and SnSe, are of great scientific interest due to their diverse possible applications ranging from thermoelectrica to phase change memory. As possible origin of these remarkable properties, the interatomic bonding is center of an ongoing scientific debate. All binary group IV-VI compounds exhibit an average number of five valence electrons per lattice site similar to elemental Sb. The crystal structures differ for the different compounds: whereas SnTe exhibits a cubic rocksalt structure, GeTe and SnSe have a rhombohedral or orthorhombic structure, respectively. The crystal structure of both GeTe and SnSe could either be viewed as layered structure or as distorted cubic structure, which would be associated with either (polar-)covalent or multicenter bonding. LaSb exhibits an ideal cubic rocksalt structure, like SnTe, but has four valence electrons per lattice site on average. Therefore, it is an ideal material for comparison of bonding characteristics with SnTe. To study the bonding characteristics of the IV-VI chalcogenides SnTe, GeTe and SnSe as well as LaSb, extended X-ray absorption fine structure spectroscopy (EXAFS) was performed at the respective pnictogen and chalcogen K-edges at ten different temperatures ranging from 10 K to 295 K at beamline P65 of PETRA III at DESY in Hamburg. Interatomic force constants up to the 4$^{th}$ nearest neighbor were extracted from the temperature dependence of the mean square relative displacement $\sigma^2$ using a correlated Einstein model. The behavior of the force constants with increasing interatomic distance is compared for the IV-VI compounds and LaSb and to other experimental and theoretical force constants reported in the literature. In relation with the distortion of the crystal structure this contributes to the fundamental understanding of the bonding characteristics in IV-VI compounds as potential origin of their remarkable properties.
