... Electrical studies of quaternary solid solutions Pb 1-x Sn x Te 1-y Se y in a wide composition range revealed the phase transition, occurred at unexpectedly low x (x>0.08). It is shown that this phase transition may be associated with Sn off-center ions. ... The simple crystal structure and unique electronic properties of IV-VI narrow-gap semiconductors make them ideal objects for studying of "impurity ferroelectricity", induced in crystals by off-center ions. ...