Off-centering of impurities is an interesting phenomenon that was discovered in 1965 in KCl crystals doped with Li. ... Investigation of narrow-gap IV-VI semiconductors (mainly lead telluride PbTe and lead selenide PbSe) doped with off-center impurities (like germanium or sulfur) attracts much attention due to impurity-induced phase transitions in these incipient ferroelectrics. ... A.I. Lebedev and I.A. Sluchinskaya. ... A.I. Lebedev, I.A. Sluchinskaya, V.N. Demin and I.H. Munro. ... Russian Acad. ...