A.I. Lebedev, I.A. Sluchinskaya
Moscow State University, Vorob'evy gory, Moscow, 119992 Russia
A new method is proposed that makes it possible to determine the parameters of the three-dimensional (3D) multiwell potential of off-center atoms from extended X-ray absorption fine-structure (EXAFS) data. The main features of this approach are the expansion of the 3D potential of a cluster in a power series of an atomic displacement taking into account the restrictions imposed by the lattice-site symmetry and exact 3D integration of the distribution function in calculations of EXAFS spectra. The parameters of the multiwell potential for Ge atoms in the Sn1-xGexTe solid solution as functions of temperature and composition (77<T<300 K, x>0.4) were obtained in the classical approximation. It is shown that the anharmonic part of the potential is highly anisotropic, and the type of ferroelectric phase transition in Sn1-xGexTe is intermediate between the displacive and order-disorder transitions.