Influence of composition on the structure and conductivity of the fast ionic conductors La2/3-xLi3xTiO3 (0.03 <= x <= 0.167)

The solid solution La2/3-xLi3xTiO3 (0.03 < x < 0.167) has been investigated by powder X-ray diffraction (XRD), impedance spectroscopy (IS) and Li-7 nu clear magnetic resonance (NMR) techniques. In these samples, a change of sy mmetry 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 graduall y when the Li content increases. Two Li signals with different quadrupole c onstants are detected in Li-7 NMR spectra of orthorhombic/tetragonal perovs kites, which have been associated with two crystallographic sites. In La0.5 Li0.5TiO3 perovskite, prepared by quenching from 1300 degreesC into liquid nitrogen, an important mobility for Li was detected in Li-7 NMR spectra. In perovskites analysed, the dependence of electrical conductivity on Li cont ent 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, hut changes much more smoothly in tetragonal ones, w here vacancies distribution becomes progressively disordered. In all perovs kites analysed, the dependence of de-conductivity with temperature displays a non-Arrhenius behaviour, with activation energies of 0.39+/-0.02 and 0.2 9+/-0.02 eV in 160-250 and 250-360 K temperature ranges respectively. (C) 2 000 Elsevier Science BV. All rights reserved.