Li 3 xLa (2/3)− xTiO 3 fast ionic conductors. : Correlation between lithium mobility and structure

Abstract

Two members of the solid solution Li 3 x La (2/3)− x TiO 3 (LLTO), with x=0.06 and 0.167, have been studied by means of X-ray diffraction, nuclear magnetic resonance (NMR) and impedance spectroscopy. In the orthorhombic sample, x=0.06, lanthanum and vacancies adopt an ordered disposition in alternating ab-planes producing the c-axis doubling of the perovskite. In the tetragonal sample, x=0.167, superstructure peaks become considerably broadened, indicating the existence of an important disorder in the distribution of vacant sites. 7Li NMR spectra of ordered samples display the presence of two types of lithium with different mobility. In the orthorhombic sample, the analysis of relaxation rates T 1 −1 and T 2 −1 indicates the existence of a bidimensional motion of Li ions between 200 and 270 K, that becomes progressively tridimensional as the temperature is increased. In the tetragonal sample, a tridimensional motion of Li is favored. Conductivity measurements indicate that Li motions are strongly correlated, displaying a non-Arrhenius behavior between 160 and 500 K. Electric relaxation behavior is independent of the composition of the sample indicating that long-range motions of Li ions are not considerably affected by structural distortions.