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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 Ea = 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...