Nonequilibrium processes and ferroelectric phase transition in PbGeTe(Ga) crystals

Abstract

The impedance of single-crystal samples of PbTe(Ga) and Pb1−xGexTe(Ga) (0⩽x⩽0.095) is investigated in the frequency range from 102 to 106 Hz and temperature range 4.2–300 K. The temperature dependence of the capacitance of all the Pb1−xGexTe(Ga) samples studied exhibited two types of features. These are a pronounced peak at a temperature T=Tp, caused by a dielectric anomaly at the ferroelectric phase transition, and a characteristic strong frequency dependence of the rise in capacitance in the temperature region T<100 K. The amplitude of the low-temperature effect decreases monotonically with increasing frequency f, and for f>105 Hz the effect practically vanishes. This behavior of the capacitance at such low frequencies may be associated with charge exchange processes in the impurity subsystem. The experimentally determined value of Tp is substantially higher than the characteristic temperatures for the appearance of long-term relaxation processes, in particular, the delayed photoconductivity. Consequently, the change of the charge states in the impurity subsystem is not accompanied by dielectric anomalies of the crystal lattice as a whole, and the possible restructuring of the lattice is of a local character.