Dilatometric and refractive parameters of rubidium sulfate crystals at low temperatures

The temperature changes of the relative elongation (Δl/l0)i and refractive indices ni(T) of rubidium sulfate crystals in the region of low temperatures are investigated in the work. The linear dimensions and volume of the crystal decrease with a decrease in temperature, and the parameters (Δl/l0)i are anisotropic. The values of ni increase with decreasing temperature for all crystal physical directions, and the intersection of the curves nz(T) and nx(Т) at a temperature of Т = 85 K occurs. This indicates the existence of an optical isotropic point in this crystal, which is confirmed by independent temperature measurements of the angle between the optical axes. It is shown that rubidium sulfate crystals are optically biaxial, with the angle between the optical axes equal 2 V = 41.5° at room temperature. With decreasing temperature, it decreases almost linearly, so that at Т = 85 K the crystal changes from optically biaxial to optically uniaxial (2 V = 0°). The temperature changes of the values of electronic polarizability αi were calculated. It was shown that αi increases slightly as temperature decrease, so that in the region where the optical isotropic point exists, the values αх and αz are equal to each other.