NMR study of the quasi-reorientational dynamics of Li ions in KTaO3:Li

Abstract

Measurements of static quadrupole effects on the NMR spectrum and of nuclear spin-lattice relaxation both in the laboratory frame, ${T}_{1}^{\ensuremath{-}1}$, and in the rotating frame, ${T}_{1\ensuremath{\rho}}^{\ensuremath{-}1}$, are reported for $^{7}\mathrm{Li}$, $^{39}\mathrm{K}$, and $^{181}\mathrm{Ta}$ nuclei in KTa${\mathrm{O}}_{3}$:Li mixed crystals as a function of Li concentration and of temperature. The aim of the present investigation is to clarify the role of the Li dynamics in the low-temperature phase transition, which has been ascribed either to a dipole-glass-type condensation or to a ferroelectric transition. Both the quadrupole splitting of the $^{7}\mathrm{Li}$ NMR spectrum and the ${T}_{1}^{\ensuremath{-}1}$ of $^{7}\mathrm{Li}$ and $^{39}\mathrm{K}$ versus $T$ indicate that for all concentrations investigated, the Li-dipole dynamics is dominated by thermally activated hopping among off-center positions displaced by 1.26 \AA{} along the [100] directions. The $^{7}\mathrm{Li}$ ${T}_{1\ensuremath{\rho}}^{\ensuremath{-}1}$-versus-$T$ data and the disappearance of the $^{181}\mathrm{Ta}$ NMR signal indicate, instead, that a small degree of preferential orientation develops even at high temperature and that it decays on a much longer time scale than the thermally activated hopping. No anomaly in either ${T}_{1}^{\ensuremath{-}1}$ or in the spectrum is observed for $^{7}\mathrm{Li}$ and $^{39}\mathrm{K}$ at the transition temperature. However, a tetragonal distortion with $\frac{(c\ensuremath{-}a)}{a}<0.007$ is still consistent with the data. Finally a detailed account is given for the nonexponential recovery of the $^{7}\mathrm{Li}$ nuclear magnetization in presence of spin-lattice and spin-spin interactions of the same order of magnitude and for the expression of the correlation function in presence of the postulated two---time-scale Li dynamics.