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

The crystal structure of PbSnF4 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 PbSnF4 undergoes small, but abrupt, increases at 608(4) and 672(3) K characteristic of the alpha --> beta and beta--> gamma phase transitions. The ambient temperature ce-PbSnF4 phase possesses a tetragonal crystal structure (space group P4/nmm), derived from the cubic fluorite arrangement by ordering of the cations in the scheme PbPbSnSnPbPb... along the [001] direction. However, the Sn2+-Sn2+ layers contain essentially no F-, with the displaced anions residing in the Pb2+-Sn2+ layers and showing significant disorder, particularly at temperatures close to the upper limit of stability of the alpha 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 alpha-PbSnF4 at temperatures close to ambient is a consequence of anion diffusion within the Ph2+-Sn2+ layers, whilst those F- within the Ph2+-Ph2+ layers are immobile. At temperatures close to the melting point of the simulated system, increased transfer of anions between the various Pb2+-Pb2+, Pb2+-Sn2+, Sn2+Sn2+ layers is observed, as the system tends towards a more isotropic anion diffusion process.