Structural changes produced during heating of the fast ion conductor Li0.18La0.61TiO3. A neutron diffraction study

The doubled perovskite structure (2ap, 2ap, 2ap) of the fast ionic conductor Li0.18La0.61TiO3 was investigated between 5 and 773 K by powder neutron diffraction. The Rietveld refinement of this orthorhombic (Cmmm Space Group) perovskite showed that at low temperature, La and vacancy rich planes alternate along [001] direction, and TiO6 octahedra were out-of-phase tilted around the b-axis. As temperature increased, the octahedral tilting decreased and the structure approaches, at about 773 K, that of the tetragonal phase, a, ap, 2ap (P4/mmm Space Group). In the temperature range of the study, the La-vacancy distribution remained unchanged, but LaO12 cuboctahedra became more regular. In the tetragonal phase the elimination of this tilting favors the two-dimensional motion of lithium in alternate ab-planes of the perovskite