Physics Dept., Moscow State University, 119899, Moscow, Russia
Investigation of ferroelectric phase transitions induced in crystals by off-center ions enables to clarify the nature of the dipole-dipole interaction in ferroelectrics. IV-VI narrow-gap semiconductors are interesting materials in which a number of off-center ions was found recently.
In this work the influence of random fields, resulting from the isoelectronic and nonisoelectronic substitution of atoms, on the ferroelectric phase transitions were studied in PbTe-based solid solutions with off-center GePb, STe and SnPb ions.
The disorder accompanying the isoelectronic substitution in ternary solid solutions with Ge and S off-center ions were found to decrease strongly Curie temperature. New hysteretic phenomena and relaxations, which were absent in ternary solid solutions, were found in quaternary solid solutions in the vicinity of Tc.
In solid solutions with Sn off-center ions the isoelectronic substitution also resulted in the appearance of hysteresis and relaxations near Tc at low concentration of off-center ions (<35 at.%), but in this case the substitution led to unexpected increase of Tc. At higher Sn concentration hysteretic behaviour and relaxations disappeared.
Studies of Pb1-xGexTe and PbTe1-xSx solid solutions doped with nonisoelectronic impurities revealed phenomena similar to those observed for isoelectronic substitution, but which were manifested much stronger. All impurities lowered Curie temperature, the rate of decreasing of Tc being dependent on impurity position in a lattice and its charged state.
The observed phenomena are explained by the influence of random fields of different origin (quazielectric, Coulomb fields, local deformation) on the local susceptibility and tunneling rate of off-center ions and on the dynamics of establishment of long-range order in a system of interacting off-center ions.
This work was partially supported by Russian Fundamental Research Foundation, grant No. 93-02-14712.