Pinning effect on microwave dielectric properties and soft mode in TlInS2 and TlGaSe2 ferroelectrics

This paper presents the results of an investigation of microwave dielectric dispersion in the proper semiconductive ferroelectrics TIInS₂ and TIGaSe₂ with an incommensurate structure modulation. In these crystals there is a strongly overdamped soft ferroelectric mode, whose frequency in the vicinity of the phase transitions drops to the millimetre wave region and causes dielectric microwave dispersion plus a high contribution to the static dielectric permittivity. Within the incommensurate phase crystal defects, such as impurities, cause pinning of the soft mode. Because of this pinning effect the phason frequency increases. Pinning also changes the dynamical dielectric properties and the contribution of the phason and amplitudon to the static permittivity.