Neutron diffraction structure determination of the high-temperature form of lithium tritantalate, H [sbnd]LiTa 3O 8

Abstract

The crystal structure of H LiTa 3O 8 has been reexamined by electron and neutron diffraction techniques. Neutron Weissenberg and electron diffraction photographs show that the space group of the compound is Pmmn and not Pmma as determined previously by X-ray diffraction techniques. There are eight molecules in the unit cell of lattice parameters a = 16.718(2)A˚, b = 7.696(1)A˚, c = 8.931(1)A˚. These values show that the baxis of the new cell is doubled with respect to the parameter measured by X-rays. The structural refinement was based on 1074 independent reflections measured on a single crystal with a four-circle neutron diffractometer. The positions of all atoms, including the lithium atoms have been determined. The final R and wRfactors were 0.036 and 0.035, respectively. The eight lithium cations occupy two sets of4 fpositions (x,¼, z)of the Pmmn space group. The ordering of four lithium ions over two sets of possible positions (4 j) of space group Pmma is responsible for the doubling of the b axis. The other four Li + occupy two sets of positions (2 d) of space group Pmma. All lithium ions are surrounded by 12 oxygen atoms arranged as cuboctahedra. The large thermal vibrations found for the lithium atoms and the ionic conductivity of H LiTa 3O 8 at high temperatures are consistent with weak Li O bonding.