Neutron Diffraction Study of Quenched Li_0.3La_0.567TiO₃ Lithium Ion Conducting Perovskite

Abstract

The Li + ion conductor Li 0.3 La 0.567 TiO 3 was synthesized as a metastable phase quenched in liquid N 2 from 1300 °C, and it was studied by time-of-flight powder neutron diffraction at high-resolution (HRPD instrument, ISIS Facility, U.K.). At room temperature a tetragonal perovskite superstructure was found, with a √2 a p × √2 a p × 2 a p unit cell ( a = 5.48027(1), c = 7.75591(2) Å, and Z = 4), I 4/ mcm space group, and a 0 a 0 c - tilt system of the TiO 6 octahedra. Superlattice hkl reflections with odd l were systematically weaker and broader. By Fourier difference maps, the Li atom was located in a disordered position inside the La vacancy hollow, close to the bottleneck site at the center of the Ti 4 O 4 square window. The Rietveld refinement converged to R p = 6.56%, R Bragg (even l ) = 3.26%, and R Bragg (odd l ) = 11.6%. A two-dimensional pathway of Li + ion mobility inside the structure, based on the coupled ordering of lithium and lanthanum, is proposed. At about 600 °C the unit cell transforms into a p × a p × a p, with a disappearance of superlattice reflections. A tetragonal pseudo-cubic P 4/ mmm structure ( a = 3.89644(1), c = 3.89748(2) Å, and Z = 1) was found ( T = 650 °C) and Rietveld refined to R p = 6.10% and R B = 4.86%. Lithium was located on the layer of Ti atoms rather than on that of La as in the room-temperature structure.