Title: ANISOTROPY OF CRITICAL INDICES OF FERROELECTRIC PHASE TRANSITION IN TGS CRYSTALS BY THE OPTICAL INTERFERENCE INVESTIGATION
Author(s): B.V.Andriyevsky, O.Ya.Myshchyshyn, M.O.Romanyuk (The Ivan Franko National University of Lviv, 8 Kyryla and Mefodiya Str., 79005 Lviv, Ukraine)

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 $^{\circ}$C, including the ferroelectric phase transition at $T_{\rm c}$=49 $^{\circ}$C, using the Jamen type optical interferometer. Temperature dependences of the spontaneous changes of the characteristics studied in the range of 39-49 $^{\circ}$C are fitted by the power-like low $Y\sim\tau^{2\beta}$ with doubled averaged effective critical indices $2\beta$=0.87-0.95. The $2\beta$ 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.

Comments: Figs. 6, Refs. 7, Tabs. 2.