A method was proposed for deducing the shape of the 3-dimensional multi-well potential of an off-center atom from EXAFS data. The parameters of the potential well of a Ge atom in GeTe and Sn1−xGexTe (x ≥ 0.4) were determined for classical and quantum-mechanical approximations. The potential-well depth ranged from 20 to 40meV; depending upon the Ge content, indicating that the phase transition in these crystals was intermediate in character between the displacement and order-disorder transitions. By analyzing the conditions for the use of the classical approximation, it followed that quantum effects had to be taken into account when determining the parameters of the potential well of an off-center Ge atom in Sn1−xGexTe. Quantum-mechanical calculations showed that the energy of the lower level in the vibration spectrum of the Ge atom coincided with the maximum energy in the potential well; to within some meV. The high probability of tunnelling or an over-barrier transition of the off-center atom between the potential-well minima prevented dipole reorientation from being frozen at low temperatures.

Parameters of the Potential Well of an Off-Center Ge Atom in a GeTe-SnTe Solid Solution. A.I.Lebedev, I.A.Sluchinskaya: Physics of the Solid State, 2007, 49[6], 1132-41