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Journal of Physical Studies 19(1/2), Article 1703 [6 pages] (2015)
DOI: https://doi.org/10.30970/jps.19.1703 REFRACTIVE INDICES OF UNIAXIALLY STRESSED Rb2ZnCl4 CRYSTALS AT THE PHASE TRANSITIONS REGIONV. Yu. Kurlyak, V. Yo. Stadnyk, V. B. Stahura
Ivan Franko National University of Lviv |
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We have studied the influence of uniaxial pressures upon the temperature dependences of refractive indices $n_i$ for Rb$_2$ZnCl$_4$ (TCZR) crystals, which reveal an incommensurate phase (IP) in a wide temperature range (192-302\,K). It is shown that the uniaxial pressures do not alter the character of the $n_i(T)$ curves but affect the exact $dn_i/dT$ values. The uniaxial pressures also increase the refractive indices (by $\partial n_i/\partial\sigma\simeq 2\cdot 10^{-6}$\, bar$^{-1}$ on average). The nonlinear changes in the $n_i(T)$ dependences are characteristic for the ТCZR crystals in the commensurate phase (CP) and the IP, although these changes remain linear in the parent phase. Moreover, the nonlinearity increases with increasing temperature so that it turns out to be larger in the IP, when compared with the СP. We have proven that the IP-to-CP phase transition is accompanied by insignificant jump-like changes $n_i$ and a temperature hysteresis ($\sim 3$\,K). The uniaxial pressure decreases the hysteresis value by $\Delta T = 0.2\ldots 0.4$\,K. We have assumed that the application of the uniaxial mechanical pressures to the crystals staying in their metastable state changes its stability, deforms the soliton structure and accelerates (or slows down) relaxation to the equilibrium state. Different contributions to the temperature changes of the refractive indices are distinguished. They originate from the electrooptic and elastooptic effects, as well as from the order parameter. In our opinion, the anomalous behavior of the contributions linked with the order parameter and the spontaneous strains can be explained by taking into account the order parameter contributions proportional to its 4th and 6th powers in the thermodynamic potential. We have estimated the corresponding critical index ($\beta= 0.31 \pm 0.02$). Its insignificant changes imposed by the uniaxial pressures ${\sigma }_m$ are of the order of measurement and calculation errors.
PACS number(s): 78.20.Ci, 78.20.Fm, 71.20.Ps