Influence of composition on the structure and conductivity of the fast ionic conductors La 2/3− xLi 3 xTiO 3 (0.03≤ x≤0.167)

Abstract

The solid solution La 2/3− x Li 3 x TiO 3 (0.03< x<0.167) has been investigated by powder X-ray diffraction (XRD), impedance spectroscopy (IS) and 7Li nuclear magnetic resonance (NMR) techniques. In these samples, a change of symmetry from tetragonal to orthorhombic is observed when the lithium content is decreased below x=0.06. Structural modifications produced are mainly due to cation vacancies ordering along the c-axis, which disappear gradually when the Li content increases. Two Li signals with different quadrupole constants are detected in 7Li NMR spectra of orthorhombic/tetragonal perovskites, which have been associated with two crystallographic sites. In La 0.5Li 0.5TiO 3 perovskite, prepared by quenching from 1300°C into liquid nitrogen, an important mobility for Li was detected in 7Li NMR spectra. In perovskites analysed, the dependence of electrical conductivity on Li content departs from that expected on the basis of a random distribution of La and Li in A sites. Dc conductivity increases quickly with the Li content in orthorhombic samples, but changes much more smoothly in tetragonal ones, where vacancies distribution becomes progressively disordered. In all perovskites analysed, the dependence of dc-conductivity with temperature displays a non-Arrhenius behaviour, with activation energies of 0.39±0.02 and 0.29±0.02 eV in 160–250 and 250–360 K temperature ranges respectively.