On isotropic states in doped K$_{2}$SO$_{4}$ crystals

The temperature, spectral and pressure dependences of the birefringence $\Delta n_{i}$ 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 $\Delta n_i$ and lead to an increase in the temperature dependence of $\Delta n_{i}(T)$. 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 $\Delta n_{i}$ 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.