Temperature dependence of IR absorption spectra of the K$_3$Na(CrO$_4$)$_2$ ferroelastic

This paper reports on the first study of IR absorption spectra of K$_3$Na(CrO$_4$)$_2$ crystals within a broad temperature interval 300-4 K. Significant changes with temperature have been observed in the region of high-frequency internal vibrations $\nu_3$ and $\nu_1$ of the CrO$_4$ group (800–1000 cm$^{-1}$), which consisted in the appearance of new features in the IR spectrum at temperatures below $T_{c1}\approx$ 230 K and $T_{c2}\approx$ 150 K. These changes derive from the presence of several phase transitions, $P\bar3 m1\to C_2/m\to C_2/c$, occurring in the ferroelastic under study. The results obtained fit to our earlier findings dealing with Raman light scattering in K$_3$Na(CrO$_4$)$_2$ single crystals.