87RbNMR study of phase transitions below room temperature in aLiK0.9Rb0.1SO4mixed crystal

Abstract

${}^{87}\mathrm{Rb}$ NMR spectra in a mixed ${\mathrm{LiK}}_{0.9}{\mathrm{Rb}}_{0.1}{\mathrm{SO}}_{4}$ single crystal have been investigated in the temperature range from 300 to 80 K. From the electric field gradient analysis, one can suggest that the Rb impurity replaces the K site in the mixed crystal. The temperature dependence of the central NMR frequency of ${}^{87}\mathrm{Rb}$ $(I=\frac{3}{2})$ demonstrates the phase transition at 255 K which occurs without a change of the ${\mathrm{Rb}}^{+}$ site symmetry. In the temperature range from 215 to 176 K, the NMR result shows a mixed phase, our tentative proposal. Another phase transition below 140 K indicates a sudden change in symmetry at the rubidium site in the crystal and a phase transition to a new phase with lower symmetry with the formation of ferroelastic domains. These phase transition temperatures are different from the ${}^{39}\mathrm{K}$ NMR study in the pure ${\mathrm{LiKSO}}_{4}$ and are not in agreement with an optical study in ${\mathrm{LiK}}_{1\ensuremath{-}x}{\mathrm{Rb}}_{x}{\mathrm{SO}}_{4}$ $(x=0.1,0.2).$ From the angular dependence of the second order quadrupole shift of the central transition of ${}^{87}\mathrm{Rb}$ NMR, the quadrupole coupling constant and the asymmetry parameter are determined at 300 and 135 K, respectively. The differential scanning calorimeter results clearly show the phase transitions near 257 and 173 K, respectively, but cannot discriminate other phase transitions.