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Abstract
The paper presents the results of the microwave investigation of the dielectric dispersion in semiconductive ferroelectric T1InS2 in the vicinity of ferroelectric and lock-in transition temperature, and in the incommensurate phase over the frequency region 107 to 1011 Hz. The dielectric dispersion is caused by the strongly overdamped soft ferroelectric mode. In the paraelectric phase its frequency varies according to vs = 29 (T - Tc )1/2 GHz and softens to 106 GHz at Ti . The soft mode makes the main contribution to the static permittivity which fits the Curie-Weiss law with the Curie constant C = 6.5 . 103 K. The dielectric measurements show a crossover from displacive to order-disorder behaviour on approaching Ti . The dielectric dispersion of Debye-type in the incommensurate phase is caused by the phase mode of the soliton lattice.