A new method to determine parameters of a three-dimensional multiwell potential of noncentral atoms from EXAFS data has been proposed. The main features of this approach are the series expansion of a three-dimensional potential in a cluster in displacement degrees with allowance for constraints imposed by lattice-node symmetry and the exact three-dimensional integration of the distribution function in calculation of EXAFS spectra. Within the framework of this approach, parameters of a multiwell potential were estimated for the first time in classical approximation for a Ge atom in solid solution Sn$_{1-x}$Ge$_x$Te ($77\le T\le300$ K, $x\ge0.4$). It is shown that the anharmonic part of the potential is strongly anisotropic and that by its type a ferroelectric phase transition in Sn$_{1-x}$Ge$_x$Te is intermediate between the displacement and order—disorder transitions.
Department of Semiconductor Physics, Faculty of Physics, Moscow State University, Leninskie Gory, Moscow, 119992, Russia
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