Spectroscopic and computer modelling studies of mixed-cation superionic fluorites

Abstract

Mixed-cation superionic fluorites are exceptionally good F- ion conductors due to the mixed nature of the cation sublattice. In this study, the authors used the extended x-ray absorption fine structure (EXAFS) spectroscopy to gain insights into the local environments of the Cd and Pb cations (as a function of composition and temperature) in CdF2 (xPbF(2)) mixed-cation superionic fluorites. A high degree of disorder is shown around both cations. However, the extent of disorder is even larger around the Pb2+ in the entire series of solid solutions, with the highest disorder noted for the x = 60 mol% PbF2 sample. The large values of the Debye-Waller factors of the Pb-F interactions are consistent with the low amplitudes of their Fourier transforms. This can be correlated with relative ionic conductivities of each composition in these materials. Molecular dynamics (MD) has been used as a complementary technique on structural properties; systems were modelled at temperatures ranging from liquid nitrogen temperature, 77 K, to 300 K. Our MD results confirm that disorder is greatest at concentrations x approximate to 60 mol% PbF2.