Optical and Dilatometric Properties of (CH$\text{}_{3}$)$\text{}_{2}$NH$\text{}_{2}$Al(SO$\text{}_{4}$)$\text{}_{2}$·6H$\text{}_{2}$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.