Isotropic Points in Doped Potassium Sulphate Crystals

The temperature, spectral and baric dependences of birefringence of potassium sulfate crystals with manganese admixture are studied in the work. It was established that the introduction of an admixture changes the absolute value of Δ n i without changing the nature of the dispersion d Δ n i / d l, and also leads to an increase in the temperature dependence of Δ n i ( T ). It is shown that isotropic points (IPs) appear in the impurity crystal of potassium sulfate at temperatures T = 610 K (Δ n y = 0, IP I) and T = 688 K (Δ n x = 0, IP II), which are shifted towards lower temperatures by 7 K and 12 K, respectively, compared to the pure crystal. The analysis of temperature and baric changes in the parameters of the ultraviolet (UV) and infrared (IR) oscillators of the potassium sulfate crystal showed that they coincide in the vicinity of IP. That is, the simplest case is realized in this crystal, when the strengths of the ultraviolet and infrared oscillators in the two crystal physical directions i and j coincide. The values and dispersion of the refractive indices of the K 2 SO 4 crystal in these directions are very close and at certain temperatures or uniaxial stresses coincide over the entire spectrum, i.e., IP is "isospectral". Baric studies have shown that uniaxial loads σ x = 200 bar and σ z = 200 bar shift the dispersion curves in the direction of larger and smaller values of Δ n y , and, accordingly, the region of existence of the isotropic state shifts. It is established that the uniaxial load σ x = 200 bar reduces Δ n y so that the "isospectrality of IP" occurs at the temperature T ~ 599 K, while the load σ z = 200 bar increases Δ n y so that the "isospectrality of IP" occurs at the temperature T ~ 628 K. Using experimentally obtained temperature and spectral dependences of birefringence of crystals for different values of uniaxial compression, generalized temperature-spectral-baric diagrams of the isotropic state of potassium sulfate crystals were constructed, which allow determining the isotropic state of crystals in certain temperature, spectral and baric ranges, which may be of practical importance in the case of using this crystal as a crystal-optical sensor.