Motion on inequivalent lattice sites: NMR theory and application to LaF₃

A general theoretical treatment is developed for NMR spin-lattice (T₁ and T_{1 rho} ) and spin-spin (T₂) times for relaxation due to modulation for dipolar interactions by diffusion of atoms on inequivalent lattice sites. This extends previous calculations of T₂ to the other relaxation parameters and gives rate equations for a single nuclear species moving over several inequivalent lattice sites and interacting with other (static) nuclear species. The temperature dependences of the relaxation times are shown to have specified characteristics, such as minima in T₂ and multiple minima in T_{1 rho} . The formalism is then used to analyse data obtained for LaF₃ to evaluate details of fluorine-ion motion. Its applicability is successfully demonstrated.