Tetragonal superstructure and thermal history of Li0.3La0.567TiO3 (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 degrees C, of the Li(0.3)La(0.56)7TiO(3) ( LLTO) lithium ion conductor, and on sample II quenched in liquid N-2. Patterns were obtained at room temperature ( RT), 400 and 700 degrees C on sample I, and at 550 degrees 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 root 2a(p) x root 2ap x 2a(p) unit- cell [ a = 5.48162( 1), c = 7.74646( 1) angstrom at RT, Z = 4; a(p) is the cell edge of cubic perovskite] in all cases except 700 degrees C, where a P4/mmm ( a(p) x a(p) x 2a(p)) structure was determined and refined [ a = 3.89827( 2), c = 7.8002( 1) A, 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 a(0)a(0)c(-) 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.