Dilatometric and optical properties of (CH3)(2)NH2Ga(SO4)(2) center dot 6H(2)O crystals in paraelectric and ferroelectric phases

Investigations of the temperature dependences of the dimension Li, optical path difference D-ij=L-i(n(j)-1), and refractive indices n(j) have been car ried out for DMAGaS crystal, (CH3)(2)NH2Ga(SO4)(2) . 6H(2)O, in the range o f 115-293 K for three principal directions of the optical indicatrix (i, j = p, m, g). The minima of the temperature dependences of anisotropy degree indices of the linear thermal expansion A(alpha) and refractive indices A(n -1) at temperature T-c reflect a small disordering of the crystal in the ph ase transition region. The coefficients of spontaneous linear electrostrict ion Q(L) have been found to be almost constant in the temperature range bel ow T-c approximate to 136 K. The molecular refraction R, as a parameter of electron polarization due to high frequency light excitation, remains uncha nged in the frame of 0.2% in the temperature range of 115-293 K.