Journal of Physical Studies 18(4), Article 4701 [23 pages] (2014) DOI: https://doi.org/10.30970/jps.18.4701 REFRACTOMETRY OF UNIAXIALLY LOADED K2SO4 CRYSTALS M. O. Romanyuk1, V. Yo. Stadnyk1, V. Yu. Kurlyak1, B. Andriyevskiy2, O. S. Kushnir3, R. S. Brezvin1 1Faculty of Physics, Ivan Franko National University of Lviv, 8, Kyryla i Mefodija St., Lviv, UA--79005, Ukraine, e-mail: vasylstadnyk@ukr.net, 2Koszalin University of Technology, 2, Śniadeckich St., Koszalin, 75--543 Poland, 3Faculty of Electronics, Ivan Franko National University of Lviv, 107, Tarnavsky St., Lviv, UA--79005, Ukraine

We describe the effect of uniaxial pressures on the refractive indices $n_i$, the birefringences $Δ n_i$, and the characteristics of optical oscillators for K$_2$SO$_4$ crystals. The uniaxial compression is shown not to change the nature of the dispersion effect but to influence only its magnitudes $dn_i/dλ$ and $dΔ n_i/dλ$. It is established that the increase in $n_i$ due to the uniaxial pressure is mainly caused by changing parameters of the oscillators and their effective density. We find that a simultaneous application of the pressures can lead to the apprearance of new isotropic states in K$_2$SO$_4$ crystals. We have also found significant baric shifts of the ferroelastic phase transition (PT) point and the temperature interval where an intermediate phase exists near that PT. We have calculated spectral and temperature changes in the absolute piezooptic constants of K$_2$SO$_4$ crystals and have found intersections of $π_{13}(T)$ and $π_{31}(T)$ curves and $π_{21}(T)$ and $π_{32}(T)$ curves at the temperatures that correspond to positions of their isotropic points. The electronic band structure of K$_2$SO$_4$ is calculated, and the nature of their valence and conduction bands is ascertained. The uniaxial pressures $σ$ < 1.5 GPa applied along the principal crysatallographic directions impose an increasing bandgap $E_g$. The experimental results obtained for the refractive indices near the PT are shown to be satisfactorily described in the framework of phenomenological approach of a first fluctuation correction to the Landau theory when taking into account the influence of point structural defects on the $n_i(T)$ behavior in the vicinity of small relative temperatures. A generalized temperature-spectral-baric diagram of the isotropic state is constructed for the K$_2$SO$_4$ crystals. A method for monitoring the $Δ n_i$ magnitude in optically biaxial crystals is suggested, which is based on the measurements of the conoscopic pattern parameters.

PACS number(s): 78.20.Сі, 78.20.Fm, 77.80.-e, 71.20.Ps, 42.65.An, 42.25.Lc

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