SnK, PbL3, and BaL3 EXAFS, X-ray diffraction and 119Sn Mössbauer spectroscopic studies of ordered MSnF4 (M = Pb and Ba) fluoride ionic conductors with the α-PbSnF4 structure

The high performance fluoride ion conductors α-PbSnF4 and BaSnF4 have been studied using X-ray diffraction, 119Sn Mössbauer spectroscopy, and EXAFS. X-ray diffraction shows the unit-cell is a tetragonally distorted fluorite-type, with ordering of the metals which results in a superstructure along the c axis. The SnK, PbL3, and BaL3 X-ray absorption spectra of α-PbSnF4 and BaSnF4 as well as model compounds (SnO, β-PbF2, and BaF2) have been recorded at 300 and 77 K. Analysis of the extended fine structure (EXAFS) of α-PbSnF4 and BaSnF4 indicates they have a similar local structure around the corresponding metal atoms with average nearest neighbor distances of RSnF = 2.08(3) Å, RPbF = 2.50(3) Å and RBaF = 2.67(2) Å. The good agreement between the PbF and BaF distances derived from EXAFS with diffraction results and ion pair estimates indicates that the Pb and Ba ions determine the close-packing arrangement of the crystal structure. EXAFS shows that the local structure is much better defined around Pb and Ba than around Sn. This and the weak temperature dependence of the SnK EXAFS indicates a lower rigidity of next-neighbor fluorine shells around Sn than around Pb or Ba. In the α-PbSnF4 structural type, the metal M (Pb or Ba) is in an eightfold coordination site similar to the cubic coordination in the fluorite-type, whereas Sn is in a unique SnF5E pseudooctahedral coordination, with the lone pair E being stereoactive and making the materials strongly anisotropic. This is confirmed by the large quadrupole splitting observed in Mössbauer spectroscopy, Δ = 1.52(2) mm/s for α-PbSnF4 and 1.52(1) mm/s for BaSnF4. The mobile fluoride ions are probably disordered and widely spread over conduction paths, and they remain disordered even at low temperature when their long range motion is frozen. The structural results are discussed in relationship to proposed mechanisms for ionic conduction in these materials.