TEMPERATURE DEPENDENCE OF THE REFRACTIVE INDICES OF DOPED [K.sub.2]S[O.sub.4] CRYSTALS

• Published in: Journal of applied spectroscopy 2022-02, Vol.88 (4), p.831
• Main Authors: Stadnyk, V.Yo, Shchepanskyi, P.A, Rudysh, M.Ya, Kogut, Z.A
• Format: Article
• Language: English
• Subjects: Crystals; Equipment and supplies; Fiber optics; Structure
• ISSN: 0021-9037; 1573-8647
• DOI: 10.1007/s10812-021-01247-8

The temperature dependence of the refractive indices of [K.sub.2]S[O.sub.4] with copper impurity was investigated. It was found that introduction of the copper impurity leads to the appearance of isotropic points [intersection of the [n.sub.2](T) and [n.sub.x](T) curves at temperature T' ~ 612 K ([DELTA][n.sub.y] = 0) and [n.sub.z](T) and [n.sub.y](T) at temperature T" ~ 689 K ([DELTA][n.sub.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 ([beta] [approximately equal to] 0.17 [+ or -] 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.