Temperature Dependence of $^{7}$Li NMR Spectra in a Li$_{2}$KRb(SO$_{4})_{2}$ Single Crystal

Li$_{2}$KRb(SO$_{4})_{2}$ is a mixed crystal of LiKSO$_{4}$and LiRbSO$_{4}$. LiKSO$_{4}$ has a hexagonal symmetry at room temperature and undergoes four phase transitions at low temperature. On the other hand, LiRbSO$_{4}$ is pare-electric with a monoclinic symmetry at room temperature and then shows a phase transition above 400 K. In order to understand the microscopic details of structural phase transitions in the single crystal of Li$_{2}$KRb(SO$_{4})_{2}$, we have measured the temperature dependence of $^{7}$Li NMR spectrum at 8 T from 300 K down to 100 K. The $^{7}$Li NMR spectrum shows three resonance peaks, which is a typical shape from three nuclear Zeeman level splitting for the nuclear spin of I$=$3/2 with nuclear-quadruple interaction. The spectrum shows a different shape for 8T parallel and perpendicular to the c-axis. As temperature decreases, the spectrum shows no significant change whereas the $^{7}$Li nuclear quadrupole frequency increases monotonically.