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

Abstract

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 (T 1) decreases monotonously with increasing temperature at the temperature range studied (ca. 230–360 K), 119Sn and 207Pb T 1 bear T 1 minima. From the short 119Sn T 1 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.

High Anionic Conductive Form of PbxSn2–xF4

“…The characteristic MAS pattern was attributed to the local coordination structure of Sn in the β form; four 4f F atoms and one 2c F atom are placed one-sidedly, thus leading to the large chemical-shielding anisotropy. Detailed analysis of the 119 Sn lineshape of the β form including effects of F – motion is found in ref . To conclude, the almost same sideband patterns for the β and β + forms suggest an almost identical local coordination structure of Sn for them, which is consistent with the structure obtained by the ND experiments.…”

“…Such disorder in a long-range structure would disturb the Rietveld analysis of the ND experiment. Further, the Rietveld fitting becomes difficult because of large anisotropic thermal parameters of F atoms reflecting its facile motion and also of partial site occupancies, which enable efficient defect diffusion (ion conductivity). In this work, to confirm the local structure of β + -PbSnF 4 suggested by the analysis of the SXRD and ND results, we apply high-resolution solid-state NMR, which is a powerful means to examine the local structure without being disturbed by the long-range disorder.…”

“…and 207 Pb), and among them,207 Pb NMR would be of interest as three different Pb sites, namely, two sites in the triple Pb layer and one in the double Pb layer, are suggested. In the following NMR measurement, we examine the x = 1.21 sample as the β + -PbSnF 4 sample and compare with the results of β-PbSnF 4 29,30. Figure4compares 19 F, 119 Sn, and 207…”

Fluoride ion batteries – past, present, and future

“…66,67 This interaction also promotes conductivities within the crystallite and at the particle boundaries, with ion mobility at the macroscopic material level, which is in slight contrast to what is reported in previous studies. [68][69][70] Low activation energies result in high ionic conductivities at room temperature (Figure 10b), with values close to single crystals and better than what was found previously for fluorite type materials. 71,72 The solid electrolytes of PbxSn2-xF4 were also prepared by high-energy planetary ball-milling.…”

Preparation and Performance of Eu3+-Doped BaSnF4-Based Solid-State Electrolytes for Room-Temperature Fluoride-Ion Batteries

“…Sn-based metal fluorides such as PbSnF 4 usually have a good ionic conductivity at RT. According to the reported results, the ionic conductivities of BaSnF 4 and PbSnF 4 can be as high as 1 × 10 –3 and 1 × 10 –4 S cm –1 , respectively. − However, until now, these electrolytes have rarely been tested as solid-state electrolytes in FIBs. It has been found that the tetragonal BaSnF 4 shows an outstanding RT ionic conductivity, which is conductive to the application of energy storage systems .…”