The crystal structure of α-PbSnF4 and its anion diffusion mechanism

The crystal structure of PbSnF 4 and the nature of the anion diffusion mechanism which characterizes its high ionic conductivity have been investigated by impedance spectroscopy, powder neutron diffraction and computer simulation methods. The ionic conductivity of PbSnF 4 undergoes small, but abrupt, increases at 608(4) and 672(3) K characteristic of the and phase transitions. The ambient temperature α - PbSnF 4 phase possesses a tetragonal crystal structure (space group P 4/ nmm ), derived from the cubic fluorite arrangement by ordering of the cations in the scheme PbPbSnSnPbPb along the [001] direction. However, the Sn 2+ –Sn 2+ layers contain essentially no F − , with the displaced anions residing in the Pb 2+ –Sn 2+ layers and showing significant disorder, particularly at temperatures close to the upper limit of stability of the α phase. Computer simulations, using interionic potentials derived from first-principles calculations and containing realistic representations of polarization effects, are in good agreement with the measured ionic conductivity and successfully reproduce the experimentally determined ionic distribution. Analysis of the simulated ionic motions demonstrate that the impressive ionic conductivity of α - PbSnF 4 at temperatures close to ambient is a consequence of anion diffusion within the Pb 2+ –Sn 2+ layers, whilst those F − within the Pb 2+ –Pb 2+ layers are immobile. At temperatures close to the melting point of the simulated system, increased transfer of anions between the various Pb 2+ –Pb 2+ , Pb 2+ –Sn 2+ , Sn 2+ –Sn 2+ layers is observed, as the system tends towards a more isotropic anion diffusion process.