Temperature Dependence of the Refractive Indices of Doped K2SO4 Crystals

The temperature dependence of the refractive indices of K2SO4 with copper impurity was investigated. It was found that introduction of the copper impurity leads to the appearance of isotropic points [intersection of the n2(T) and nx(T) curves at temperature T' ~ 612 K (Δny = 0) and nz(T) and ny(T) at temperature T″ ~ 689 K (Δnx = 0)] and also to widening of the temperature range for existence of the intermediate phase and its shift toward lower temperatures. The critical index of the phase transition order parameter was determined for the doped crystal in terms of the phenomenological approach of the first fluctuation correction to the Landau theory (β ≈ 0.17 ± 0.05). It was found that the phase transition point is shifted toward low temperatures, and this can be explained at the phenomenological level by the effect of internal stresses that arise during introduction of the impurity by analogy with the effect of uniaxial or hydrostatic pressures.