It is shown that this phase transition may be associated with Sn off-center ions. The unusual enhancement of T c , induced by substitutional disorder, was explained by strong reduction of tunneling of Sn off-center ions resulting from the increase of displacement of Sn atoms when substituting the large atoms in anion sublattice by smaller ones. In the present work we shall show that similar properties are characteristic for Sn atoms in IV-VI compounds as well. Monocrystals prepared by sublimation method were also investigated. The homogeneity of samples was checked by X-ray method.
Solid curves represent computer approximation of experimental data.
Se y can be also associated with some off-center ions. Se y illustrating this effect. One can see that besides the above mentioned hysteresis effect, the curves, obtained after quick and slow cooling, differ appreciably below T c . This effect was fully reproducible and was observed on different samples with low x. Se y solid solution only two of them, Sn and Se, can be regarded as candidates for off-center ions.