LOW-TEMPERATURE PROPERTIES OF INCOMMENSURATE FERROELECTRICS (PBYSN1-Y)(2)P2SE6

The complex dielectric susceptibility epsilon = epsilon(') + epsilon(' ') of the incommensurate ferroelectrics (PbySn1-y)(2)P2Se6 (at y great er than or equal to 0.4) has been studied at various frequencies (0.5 Hz - 100 kHz) in the temperature range from 10 mK to 200 K. Three diff erent types of dielectric behaviour are distinguished at low temperatu re. Below 500mK typical glassy behaviour is observed according to the two-level system (TLS) model with a constant density of states. In the range from 0.5 K to 7 K the dielectric results are described in the T LS model with an energy dependent density of states and above 7 K the dielectric dispersion and associated maximum of epsilon('') has a ther moactivated relaxation nature. The latter is attributed to the freezin g of the relaxation dynamics of the incommensurate modulation.