... Moscow State University, 119899, Moscow, Russia The results of experimental studies of the influence of substitutional disorder, doping impurities and of mutual influence of different off-center atoms on the ferroelectric phase transitions induced in IV-VI semiconductors by off-center atoms are reviewed. Keywords: phase transition, off-center ions, substitutional disorder, random fields, impurities . ... The example of such defects is off-center ions. ... Influence of doping impurities . ...