The temperature, spectral and pressure dependences of the birefringence Δni of potassium sulfate crystals doped with 1.7% of copper have been studied. The introduction of the impurity is found to change the absolute value of Δni and lead to an increase in the temperature dependence of Δnі(Т). It is shown that the isotropic points appear in the doped crystal of potassium sulfate, and are shifted, compared with a pure crystal, towards low temperatures by 5 K and 11 K, respectively. It was established, that uniaxial stresses change the magnitude of the Δni absolute values, as well as the region of existence of the isotropic state. Generalized temperature-spectral-baric diagrams of the isotropic state of doped potassium sulfate crystal are constructed, making it possible to use them as a crystal-optical sensor.

The spectral, temperature, and pressure dependences of the birefringence Δni of LiNaSO4 crystals are studied. Dispersion Δn(λ) is found to be anomalous. The uniaxial compression leads to different in magnitude and sign changes of Δnі. The dispersion effect of Δnі changes is weakening under the influence of pressure sigma ⟘ z and increases in the case of sigma || z. The baric changes of the refractive indices are estimated. The refractive indices of the LNS crystal are found to increase under the action of uniaxial pressure, which may be due to the compression of the crystal and a change in the nature of its interatomic bonds.