Critical indices of the ferroelectric phase transition in TGS crystals

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 ~ τ²β 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.Шляхом оптичних вимірювань сегнетоелектричного фазового переходу в кристалі тригліцинсульфату за допомогою інтерферометра Жамена одержано температурні залежності оптичної рiзниці ходу, електронної сприйнятливості та лінійного розширення для трьох кристалофізичних напрямків. Температурні залежності спонтанних змін досліджуваних характеристик в області 39–49◦ C апроксимовані степеневими залежностями Y ~ τ²β з подвійними критичними індексами 2β=0.87–0.95. Відмінність 2β від одиниці пояснюється суттєвою температурною залежністю поблизу точки фазового переходу коефіцієнтів електрооптичного, оберненого п’єзоелектричного ефектів та електрострикції

Optical and Dilatometric Manifestations of Phase Transitions in K2\text{}_{2}SeO4\text{}_{4} Crystal

Temperature dependencies of the characteristic optical paths difference D(Τ) of K2SeO4 crystal in the range of 80-295 K were measured using the Jamen type interferometer. Significant anomalies of D(Τ) dependencies in the vicinity of incommensurate-paraelectric phase transition (T$\text{}_{i}$ = 130 K) are observed. On the basis of D(Τ) temperature dependencies measured experimentally and linear expansion l(Τ) known, the temperature derivative of the refractive indices dn/dT are found to be negative in the ranges above and below the temperature T$\text{}_{i}$=130 K

Optical and Dilatometric Properties of (CH3\text{}_{3})2\text{}_{2}NH2\text{}_{2}Al(SO4\text{}_{4})2\text{}_{2}·6H2\text{}_{2}O Crystals in Paraelectric and Ferroelectric Phases

Experimental investigation of the thermal linear expansion l(T) and temperature dependencies of the interference optical path difference D(T) were carried out for (CH$\text{}_{3}$)$\text{}_{2}$ NH$\text{}_{2}$Al(SO$\text{}_{4}$)$\text{}_{2}$·6H$\text{}_{2}$O crystal, in the range of 120-310 K for three crystal physical directions. Decrease in electrostriction coefficients g of crystal was observed on approaching the phase transition point T$\text{}_{c}$ ≈ 150 K in ferroelectric phase. Relative temperature changes of the lengthening δl/l and refractive index δn/(n-1) are of the same order of magnitude. Peculiarities of temperature dependence of dilatometric and optical parameters of (CH$\text{}_{3}$)$\text{}_{2}$NH$\text{}_{2}$Al(SO$\text{}_{4}$)$\text{}_{2}$·6H$\text{}_{2}$O testify for qualitative reconstruction of chemical bonds in the crystal when temperature changes in the same phase