Study of the La1/2+1/2xLi1/2-1/2xTi1-xAlxO3 (0 ≤ x ≤ 1) solid solution. A new example of percolative system in fast ion conductors

The synthesis by solid state reaction of new fast ion conductors with perovskite structure was carried out. The crystal structure and electric properties of the La1/2+1/2xLi1/2-1/2xTi1-xAlxO3 (0 ≤ x ≤ 1) solid solution were investigated by powder X-ray diffraction and impedance spectroscopy. All compositions of the La1/2Li1/2TiO3-LaAlO3 system, exhibited a single cubic perovskite structure (ac ≈ 3.87-3.79 Å; SG Pm-3m). The progressive decrease in the unit cell parameters agrees with the lower ionic radii of Al3+ in relation to Ti4+, which are allocated in the same octahedra. An upward deviation from the lineal ideal solid solution behavior described by Vegard's law was observed and it was tentatively associated with a volume excess created by solid dilution of non-isovalent cations. Structural features were deduced from Rietveld analysis of XRD patterns. Ti(Al)O6 octahedra are regular and La/vacancies are randomly distributed in A-site of the perovskite. The conductivity decreased almost four orders of magnitude with the Li content. This important decrease on the conductivity was attributed to the charge carrier (Li+) decrease and the blockade of the perovskite conduction pathways by La ions, according to a three dimensional percolative process. In consequence we present here a new example of percolative system of ionic conductors and the results confirm the important role played by effective vacant A-sites, neff = [Li] + nA, on Li conductivity of this fast ion conductors family with perovskite structure.