Tetragonal superstructure and thermal history of Li_0.3La _0.567TiO₃ (LLTO) solid electrolyte by neutron diffraction

High-resolution time-of-flight neutron powder diffraction data (ISIS facility, Rutherford Appleton Laboratory, U. K.) were collected on sample I, prepared by slow cooling from 1300 °C, of the Li_0.3La _0.567TiO₃ (LLTO) lithium ion conductor, and on sample II quenched in liquid N₂. Patterns were obtained at room temperature (RT), 400 and 700 °C on sample I, and at 550 °C on sample II, studied at RT in previous work. The structure was resolved and Rietveld refined in the tetragonal P4/nbm space group with √2a_p × √2a _p × 2a_p unit-cell [a = 5.48162(1), c = 7.74646(1) Å at RT, Z = 4; a_p is the cell edge of cubic perovskite] in all cases except 700 °C, where a P4/mmm (a_p × a_p × 2a_p) structure was determined and refined [a = 3.89827(2), c = 7.8002(1) Å, Z = 2]. In both space groups lanthanum ordering over two La1 and La2 sites is present, but only in P4/nbm can an additional anti-phase octahedral tilting of type a⁰a⁰c^- be observed, similar to what occurs in the I4/mcm RT metastable phase of the quenched sample. The two distortions are associated to hkl superlattice reflections with h + k even, l odd, and h + k odd, l odd, respectively. Lithium atoms were located in highly disordered sites within the less populated La1 hollows, with short Li-Li′ distances accounting for the high ionic mobility. Possible ordering schemes indicate a mainly (001) two-dimensional mechanism of ion hopping among Li sites.