Journal of Physical Studies 28(1), Article 1701 [7 pages] (2024) DOI: https://doi.org/10.30970/jps.28.1701 CHANGES IN THERMAL AND REFRACTIVE PARAMETERS OF THE SULFATE GROUP CRYSTALS IN THE REGION OF THE PHASE TRANSITION P. A. Shchepanskyi1 , V. Yo. Stadnyk1 , I. A. Pryshko1, R. S. Brezvin1 , O. R. Onufriv2, Z. Kohut{3,4}  1Ivan Franko National University of Lviv, 19, Drahomanov St., Lviv, UA–79005, Ukraine, e-mail: vasylstadnyk@ukr.net 2Ukrainian National Forestry University, 103, General Chuprynka St., Lviv, UA–79057 Ukraine, 3Częstochowa University of Technology, ul. J. H. Dąbrowskiego, 69, 42-201 Częstochowa, Poland, 4Lviv Polytechnic National University, 5, Ustyyanovych St., Lviv, UA–79013, Ukraine Received 03 November 2023; in final form 03 December 2023; accepted 20 December 2023; published online 12 February 2024

Temperature changes of linear expansion and refractive indices of rubidium sulfate (RS) Rb$_2$SO$_4$ crystals were investigated in the work. It was established that the linear dimensions increase as the temperature increases. For the $Z$-direction (the direction of occurrence of ferroelastic deformation) these changes are significant. The temperature dependence of the spontaneous deformation $δχ_z$ was calculated, and it was found that the value of $δχ_z$ decreases parabolically as the temperature increases.

DTA was carried out and derivatogramm and thermogravimetrogramm of RS crystal were obtained. At the temperature of 923 K, a clear anomaly was found, which corresponds to the position of the phase transition (PT) point, which is endothermic in nature. In the temperature range of 625-738 K, a loss of the mass of the sample was detected, which was caused by the thermal decomposition of ОН$_3^{+}$ ions and the emission of Н$_2$ and О$_2$ molecules.

It was established that the temperature changes of the refractive indices $n_i$ of Rb$_2$SO$_4$ crystal decrease non-linearly as the temperature increases, and $|dn_z/dT| > |dn_x/dT| ≥ |dn_y/dT|$. During the PT, no jump-like changes in $n_i(T)$ were detected, which is characteristic of PT of the 2nd kind. The values of spontaneous changes in refractive indices $δ n_i^{\rm s}(T)$ were obtained. These are similar in behavior to the experimentally obtained dependence $χ_i^{\rm s}(T)$ and can be described by a parabolic dependence.

Using the obtained experimental values, the critical index of changes $δ n_i^{\rm s}(T)$ was estimated ($β = 0.21$), which corresponds to the values derived from Landau's thermodynamic theory for PT. It is assumed that the main cause of the detected changes in the vicinity of the PT is defects of the “random temperature” type.

The temperature change of the electronic polarizability $α(T)$ of the Rb$_2$SO$_4$ crystal was estimated. It characterizes the degree of displacement of the structural elements from the equilibrium position under the action of the external electric field of the incident light wave. In the PT region, the dependence of $α(T)$ has a complex anomalous character: it slightly decreases as the temperature increases, and after the PT, the value of $α(T)$ practically does not change with temperature.

Key words: thermal expansion, spontaneous deformation, differential thermal analysis, refractive index, critical index, electronic polarizability.

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