Temperature Dependence of the Li NMR Spectrum and Atomic Motion in LiNbO3

Electric quadrupole coupling constants and linewidths of the Li NMR spectrum of a single crystal of LiNbO₃ were measured at temperatures between 24 and 680°C and are interpreted in terms of the motion of Li. The linear increase of the quadrupole coupling constant is unusual and is assumed to be due to the anisotropic vibration of Li. A simple expression for the temperature variation is derived which depends only on ${\mathrm{〈z}}^2{\mathrm{〉\hspace{0.17em}-\hspace{0.17em}(x}}^2〉$ and ${\partial }^4{\mathrm{V\hspace{0.17em}/\hspace{0.17em}\partial }}^{4}z$ , where ${\mathrm{〈z}}^2〉$ is a mean square amplitude of vibration of Li, and $V$ is the crystal electric potential at a Li site. ${\partial }^4{\mathrm{V\hspace{0.17em}/\hspace{0.17em}\partial z}}^4$ was computed by an ionic lattice summation. It is suggested that a linear increase of the quadrupole coupling constant is characteristic of a light atom that vibrates anisotropically. Analysis of the linewidth data, assuming that Li atoms diffuse, gives $D_0{\text{\hspace{0.17em}=\hspace{0.17em}1.8\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−5}}{\mathrm{m}}^2{\sec }^{\text{−1}}$ and $\text{ε\hspace{0.17em}=\hspace{0.17em}1.62}\mathrm{eV}$ .