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Experimental investigation of the thermal linear expansion 1(T) and temperature dependencies of the interference optical path difference D(T) were carried out for (CH 3 )2 NH2Al(SO 4 )2 • 6H2O 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 Tc 150 K in ferroelectric phase. Relative temperature changes of the lengthening 51/1 and refractive index δn/(n -1) are of the same order of magnitude. Peculiarities of temperature dependence of dilatometric and optical parameters of (CH3)2NH2Al(SO4)2.6112O testify for qualitative reconstruction of chemical bonds in the crystal when temperature changes in the same phase.

Investigations of the temperature dependences of the dimension L i , optical path difference D ij ¼ L i (n j À 1), and refractive indices n j have been carried out for DMAGaS crystal, (CH 3 ) 2 NH 2 Ga(SO 4 ) 2 Á 6H 2 O, in the range of 115-293 K for three principal directions of the optical indicatrix (i, j ¼ p, m, g). The minima of the temperature dependences of anisotropy degree indices of the linear thermal expansion A a and refractive indices A nÀ1 at temperature T c reflect a small disordering of the crystal in the phase transition region. The coefficients of spontaneous linear electrostriction Q L have been found to be almost constant in the temperature range below T c % 136 K. The molecular refraction R, as a parameter of electron polarization due to high frequency light excitation, remains unchanged in the frame of 0.2% in the temperature range of 115-293 K.

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The dispersions of the principal refractive indices n(λ) of NH4HSeO4 single crystals in the range of 400 to 700 nm are measured at room temperature. The low‐energy edge ωe of the fundamental absorption of the crystal is determined and the temperature dependences of optical and interference light path differences (retardations) related to the susceptibility η and to the birefringence Δn are studied in the temperature range of 220 to 300 K. Different behaviors of the above parameters below and above the temperature Ti = 262 K are found.