Cross relaxation and atomic motion in LiNbO2.

Abstract

Nuclear magnetic relaxation rates and linewidths are reported for the $^{7}\mathrm{Li}$ and $^{93}\mathrm{Nb}$ resonances in the layered semiconducting compound ${\mathrm{LiNbO}}_{2}$. $ sup 7---Li line narrowing and ${\mathit{T}}_{1\mathrm{\ensuremath{\rho}}}$ measurements provide unambiguous signatures of Li diffusive motion and the Li long-range hopping rate is determined as a function of temperature. Rotating-sample experiments show that the frequency dependence of the Li spin-lattice relaxation rate at low resonance frequencies results from cross relaxation to the quadrupole-broadened $^{93}\mathrm{Nb}$ spins. A crucial aspect of the cross relaxation is the large frequency variations of the $^{93}\mathrm{Nb}$ transitions resulting from the diffusion of Li vacancies. $^{93}\mathrm{Nb}$ relaxation rates are reported and shown to support both the cross relaxation and the Li hopping interpretations. In high magnetic fields, cross relaxation is suppressed and the $^{7}\mathrm{Li}$ and $^{93}\mathrm{Nb}$ spin-lattice relaxation rates differ by three orders of magnitude. The frequency and temperature dependences of these high field relaxation rates indicate that the relaxation mechanisms are not associated with diffusive Li motion. For both Li and Nb, nearly linear temperature dependences of ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ result from interaction with conduction electrons. The electrons are believed to be due to the acceptorlike behavior of Li vacancies. The much slower $^{7}\mathrm{Li}$ relaxation rate ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ reflects the much smaller electron density (at the Fermi surface) at the Li sites than at the Nb sites. Sample-to-sample variations in the Li ion mobility and in the Nb ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ are believed to reflect variations of the Li deficiency of the material.