Visnyk of the Lviv University. Series Physics 60 (2023) с. 78-89 DOI: https://doi.org/10.30970/vph.60.2023.78 Isotropicity in rubidium sulphate crystals I. Pryshko, V. Stadnyk, P. Shchepanskyi, L. Karpliuk, N. Ftomyn

The work is devoted to the study of optical isotropic points in Rb₂SO₄ crystal. The isotropic points here imply an increasing in the symmetry of the optical indicatrix when the spectral range or temperature changes. The very transition through isotropic point is accompanied by a change in the sign of birefringence of the crystal, that is, its inversion takes place. The crystals for study were obtained by the method of slow evaporation of water from repeatedly recrystallized solution of pure rubidium sulfate Rb₂SO₄ salt. The crystal structure of obtained samples was studied by means of X-ray powder diffraction spectra. Measurement of refractive indices n_i(T) and birefringence \Delta n_i(T) of the crystal were carried out in the range from room temperature to liquid nitrogen temperature of 77 K for three polarizations of light E \mid\mid X, E \mid\mid Y and E \mid\mid Z. It was found that the dependences of n_i(T) increase almost linearly with decreasing temperature, and |dn_z/dT|>|dn_x/dT|\geq|dn_y/dT|. For a light wavelength of \lambda=500 nm, the intersection of n_z(Т) and n_y(Т) curves at room temperature, and the intersection of n_z(Т) and n_х(Т) curves at a temperature of 85 K were found, which approve the existence of isotropic points in this crystal. It is established that the dispersion of \Delta n_i(\lambda) is normal d(\Delta n_і)/d\lambda<0, and d(\Delta n_х)/d\lambda>d(\Delta n_z)/d\lambda>d(\Delta n_y)/d\lambda. The temperature transformations of the optical indicatrix of the rubidium sulfate crystal in the case of a temperature decreasing, namely transitions from an optically biaxial to an optically uniaxial state, are analyzed. Since the second isotropic point in Rb₂SO₄ exists in the region of low temperatures, this crystal can be used as a working element of a crystal-optical temperature sensor in the temperature range of 75-90 K. The influence of uniaxial pressure on the birefringent properties of the crystal, as studied using a special attachment to a nitrogen cryostat, are also analyzed in this work. The effect of uniaxial compression \sigma_m on changes of \Delta n_y (T, \lambda) is established and their significant sensitivity is revealed. Herewith, \sigma_m do not change the character of these curves, but only change the absolute value of \Delta n_i.

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