Two dimensional fluoride ion conductor RbSn2F5 studied by impedance spectroscopy and 19F, 119Sn, and 87Rb NMR

RbSn2F5 is a two-dimensional fluoride ion conductor. It undergoes a first-order phase transition to a superionic state at 368 K. The structure of the low temperature phase has been determined from the Rietveld analysis of the X-ray powder diffraction. The dynamic properties of the fluoride ions in RbSn2F5 have been studied by impedance spectroscopy and solid state NMR. The dc ionic conductivity of this sample shows an abrupt increase at the phase transition temperature. We have obtained the hopping frequency and the concentration of the charge carriers (F- ions) at different temperatures from the analysis of the conductivity spectra using Almond-West formalism. The estimated values of the charge carriers' concentration agree well with that determined from the structure and were found to be independent of temperature. The relatively small value of the power-law exponent, n approximate to 0.55, supports the two-dimensional property of the investigated material. Furthermore, F-19 NMR with simulation has suggested the diffusive motions of the fluoride ions between different sites. In contrast, Sn-119 and Rb-87 NMR spectra below 250 K supported the intrinsic disordered nature due to the random distribution of the fluoride ion vacancies.