Rb-87 NMR study of phase transitions below room temperature in a LiK0.9Rb0.1SO4 mixed crystal

Rb-87 NMR spectra in a mixed LiK0.9Rb0.1SO4 single crystal have been invest igated in the temperature range from 300 to 80 K. From the electric field g radient analysis, one can suggest that the Rb impurity replaces the K site in the mixed crystal. The temperature dependence of the central NNR frequen cy of Rb-87 (I=3/2) demonstrates the phase transition at 255 K which occurs without a change of the Rb+ site symmetry. In the temperature range from 2 15 to 176 K, the NMR result shows a mixed phase, our tentative proposal. An other 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 wit h lower symmetry with the formation of ferroelastic domains. These phase tr ansition temperatures are different from the K-39 NMR study in the pure LiK SO4 and are not in agreement with an optical study in LiK1-xRbxSO4 (x=0.1, 0.2). From the angular dependence of the second order quadrupole shift of t he central transition of Rb-87 NMR, the quadrupole coupling constant and th e asymmetry parameter are determined at 300 and 135 K, respectively. The di fferential scanning calorimeter results clearly show the phase transitions near 257 and 173 K, respectively, but cannot discriminate other phase trans itions.