Silent central modes of the superionic conductor LiKSO4 studied by means of hyper-Raman scattering: Evidence of coupling between lithium-ion conduction and sulfate-ion rotation.

Abstract

Relaxational central modes due to the rotational Brownian motion of the sulfate ions are measured by hyper-Raman scattering in ${\mathrm{LiKSO}}_{4}$ crystal. The ${\mathit{B}}_{1\mathit{u}}$ relaxational mode is the rotational mode about the c axis. This is a silent mode, which is inactive for Raman and infrared measurements. The infrared-active ${\mathit{E}}_{1\mathit{u}}$ relaxational modes are also found, which are the rotational modes about the a and b axes. Since the anisotropy of the ${\mathrm{Li}}^{+}$-ion conductivities is well elucidated by the ratio of ${\mathit{B}}_{1\mathit{u}}$- and ${\mathit{E}}_{1\mathit{u}}$-rotational relaxation times, the coupling between sulfate-ion rotation and ${\mathrm{Li}}^{+}$-ion migration is strongly supported. Relaxational central modes are also found in Raman scattering. A comparison between the Raman and hyper-Raman spectra shows that interactions between sulfate ions are strong in rotations about the a and b axes. However, they are weak for rotations about the c axis.