Temperature-and-Spectral Deformations of the Optical Indicatrix of Rubidium Sulphate Single Crystal

The paper deals with newly investigated temperature-and-spectrum dependences of principal birefringences (Δ $n_{i}$(λ,T)-i=X, Y, Z; λ=250÷850 nm; T = 4.2-1100 K), and temperature dependence of the optic-axial angle (2V(T) - T=4.2-1100 K) for two laser lines 632.8 nm and 441.1 nm of $Rb_{2}SO_{4}$ crystals. Based on the temperature dependence of the optic-axial angle, the second point of birefringence sign inversion has been found. Newly obtained is the temperature-and-spectrum diagram of the uniaxial state of the optical indicatrix at the "isotropic points" and in the hexagonal phase of $Rb_{2}SO_{4}$. Based on the temperature behavior of the birefringence and optic-axial angle it was supposed that hexagonal-orthorhombic phase transition in rubidium sulfate can be a second-order phase transition. Found is the inversion of birefringence dispersion in the X and Y directions at 400 K and 620 K, respectively.