Dilatative and refractive properties of diglycine nitrate crystals in the range of phase transition

Experimental investigation of the thermal linear expansion L(T) and temperature dependences of the interference optical path difference D(T) have been conducted for ferroelectric diglycine nitrate (DGN) crystals, (NH2CH2COOH)2HNO3, for three principal directions of optical indicatrix in the range of 130–295 K including the phase transition point Tc=206 K. On the basis of the experimental data L(T) and D(T) the analogous dependences of variable part of refractive index n(T)−1, electron refraction R(T), degrees of dilatative and refractive anisotropies, AΔL/L and An−1 have been calculated. Minima of the temperature dependences of spontaneous electrostriction coefficients Qij(s)(T), anisotropy degrees of dilatative (AΔL/L) and refractive (An−1) characteristics close to Tc temperature show the relative disordering for DGN crystal in the temperature range far from Tc. The temperature dependence of the electron refraction R(T) of the crystal testifies the increase of Lorentz–Lorenz dipole–dipole interaction with decreasing temperature in the wide temperature range 295–130 K.