¹⁹F/¹¹⁹Sn/²⁰⁷Pb NMR Studies on Ion Dynamics in Tetragonal PbSnF₄: Spectroscopic Evidence for Defect-Driven Conductivity

Ion dynamics in tetragonal PbSnF₄, which shows high conductivity ∼10^–3 S cm^–1 at room temperature, was examined by using ¹⁹F/¹¹⁹Sn/²⁰⁷Pb solid-state NMR at 7 and 14 T. While the observed temperature dependence of ¹⁹F spin–lattice relaxation time (T₁) decreases monotonously with increasing temperature at the temperature range studied (ca. 230–360 K), ¹¹⁹Sn and ²⁰⁷Pb T₁ bear T₁ minima. From the short ¹¹⁹Sn T₁ minimum value at 14 T (ca. 0.85 ms) and its dependence on the strength of the static magnetic field, it was concluded that the relaxation mechanism of ¹¹⁹Sn is fluctuation of the chemical-shift anisotropy (CSA). The activation parameters for the motion responsible for the CSA relaxation were found to be τ₀ = (8.8 ± 3.1) × 10^–15 s and E_a = 26.9 ± 0.1 kJ/mol. We showed that mutual exchange of two F^– ions does not affect the size and orientation of the CSA tensor and thus cannot be the CSA-relaxation mechanism. As a mechanism to affect the CSA tensor, we invoked exchange between a F^– ion and a defect (the defect-diffusion motion). As the activation energy of the defect-diffusion motion is consistent with that obtained from temperature-dependent conductivity (∼28 kJ/mol), we concluded the defect-diffusion motion is the motion responsible for ion conduction.