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 n_x(T) curves at temperature T' ~ 612 K (Δn_y = 0) and n_z(T) and n_y(T) at temperature T″ ~ 689 K (Δn_x = 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.

研究了 K ₂ SO ₄的折射率与铜杂质的温度依赖性。发现铜杂质的引入导致各向同性点的出现[n ₂ (T) 和n _x (T) 曲线在温度T' ~ 612 K (Δn _y = 0) 和n _z (T ) 和 n _y (T) 在温度 T″ ~ 689 K (Δn _x= 0)] 以及中间相存在的温度范围的扩大及其向较低温度的转变。根据朗道理论的第一次波动校正的现象学方法（β ≈ 0.17 ± 0.05），确定了掺杂晶体的相变阶参数的临界指数。发现相变点向低温移动，这可以在现象学水平上通过在引入杂质过程中产生的内应力的影响来解释，类似于单轴或静水压力的影响。