Phase Transition in Impurity Crystals of Potassium Sulfate: Refractive Parameters

The temperature changes of birefringence and refractive indices of the impurity crystal of potassium sulfate were studied. It is found that the curves $\Delta \boldsymbol{n}_{i}(\boldsymbol{T})$ and ni $(\boldsymbol{T})$ do not change qualitatively in the case of doping, and the experimental dependences of $\boldsymbol{n}_{i}(\boldsymbol{T})$ in the region of phase transition (PT) can be well described within the framework of the phenomenological approach of the first-order fluctuation correction to Landau theory. The introduction of the Cu2+ impurity (1.7 %) is found to cause a shift in the position of the PT point towards lower temperatures by ~ 1.4 K and a slight change in the magnitude of the $\boldsymbol{n}_{i}(\boldsymbol{T})$ and $\Delta \boldsymbol{n}_{i}(\boldsymbol{T})$ curves jump during PT. It is assumed, that due to an increase in structural units in the unit cell, the presence of copper impurity leads to appearance of internal stresses similar to the influence of uniaxial or hydrostatic pressures.