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, $T_1^{-1\mathrm{}}$, and in the rotating frame, $T_{1\rho }^{-1\mathrm{}}$, 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 $T_1^{-1\mathrm{}}$ 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 Å along the [100] directions. The ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ $T_{1\rho }^{-1\mathrm{}}$-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 $T_1^{-1\mathrm{}}$ or in the spectrum is observed for ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^{39}{}_{}{}^{}\mathrm{K}$ at the transition temperature. However, a tetragonal distortion with $\frac{\mathrm{}(c-a)\mathrm{}}{a}<0.007\mathrm{}$ 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.