On the low-temperature isotropic point in Rb₂SO₄ crystals

In this work, Rb₂SO₄ single crystals of good optical quality were synthesized and the spectral (300–700 nm) and temperature (300–77 K) dependencies of birefringence were experimentally investigated. It was found that with a temperature decreasing, at a temperature of T = 85 K and a light wavelength of λ = 500 nm, the birefringence value in the Y direction is zero (Δn_y = n_z – n_х = 0), which testifies to the existence of a new isotropic point. The influence of uniaxial compression on changes in Δn_y (Т, λ) was studied, and their significant sensitivity was revealed. The temperature-baric and spectral-baric changes of the isotropic point position were analyzed, based on which the temperature-spectral-baric diagram of the isotropic state of the crystal was constructed, which will be of practical importance for the use of crystal as an active element in a crystal-optical temperature sensor for a low-temperature region. Based on the Δn_y (Т, λ, σ) dependencies, the combined piezo-optic coefficients of the crystal were estimated, and it was assumed that not only thermal but also mechanical deformation of the optical indicatrix takes place in the vicinity of the isotropic point.