Temperature dependences of the optical path difference, variable part of the refractive index and thickness of triglycine sulphate crystal for three crystallophysic directions are studied in the temperature range of 39-70• C, including the ferroelectric phase transition at T c =49• C, using the Jamen type optical interferometer. Temperature dependences of the spontaneous changes of the characteristics studied in the range of 39-49• C are fitted by the power-like low Y ∼ τ 2β with doubled averaged effective critical indices 2β=0.87-0.95. The 2β values being different from the unity is explained by the essential temperature dependence of the coefficients of electrooptic, inverse piezoelectric and electrostriction effects in the range close to the phase transition point.

Temperature dependences of the optical path difference by the variable part of refractive index D = (n − 1)l and the components ε ′ and ε ′′ of complex dielectric permittivity for NH 4 HSeO 4 single crystal are studied in the temperature range of 220-300 K. Anomalous behaviour of these dependences in the range of incommensurate phase 250-262 K was observed. Analysis of the results of dielectric investigation predicts a considerable decrease of the coefficient of elastic stiffness C in the narrow temperature range of 256-260 K at the transition from the paraelectric to incommensurate phase.

Based on solving the Kramers-Kronig integral equation, binding the spectrum of refractive index n(ω) of dielectric experimentally measured in the range of transparency (ω) and its spectrum of absorption index k(ω ′ ) in the fundamental range (ω ′ ), the dispersion analysis of these parameters has been proposed. The elaborated method has been applied to the triglycine sulphate crystal, in which the display of valence electrons interaction has been established, and the temperature dependence of the characteristic parameter of this interaction has been studied in the region including the ferroelectric phase transition at T c = 49• C.

Temperature dependencies of retardation, electron susceptibility and linear thermal expansion for three crystal-physic directions are obtained by means of optical investigations of the ferroelectric phase transition in TGS crystal using the James-type interferometer. Temperature dependencies of the spontaneous changes of the characteristics studied in the 39-49 • C range are fitted by the power low Y ∼ τ 2β with double critical indices 2β=0.87-0.95. Difference of 2β values from the unity is explained by the essential temperature dependence in the range close to the phase transition point for the coefficients of electrooptic, reversed piezoelectric and electrostriction effects.