New arguments in support of the off-center positions of the tin atoms in Pb_1-xSn_xTe_1-ySe_y and Pb_1-xSn_xTe_1-yS_y

Investigations of the electrical properties of PbSnTeS, PbSnTeSe, and PbSnSeS solid solutions confirmed an earlier hypothesis that the positions of the tin atoms were off-center and that the tunnel motion of these atoms were suppressed in the anion sublattice. In the case of PbSeTeS, containing two off-center impurities Sn and S, the ranges of compositions in which the ferroelectric phase transition was induced by the off-center Sn and S impurities were separated by a region with a disordered dipole structure.