Effect of quenching on structure and antiferroelectric instability of La(2−x)/3LixTiO3 compounds: a Raman study

A Raman study of the La(2−x)/3LixTiO3 ionic conductors has been performed as a function of temperature, lithium content and synthesis cooling rate. Irrespective of thermal treatment or lithium content, the spectrum can always be explained within a tetragonal symmetry (space group P4/mmm), which suggests that, at a local scale of several unit cells, some cation ordering persists even in samples submitted to quenching conditions. The mode involving mostly in-plane titanium vibration softens on lowering temperature, though a phase transition never occurs. The effect is attributed to the tendency of these double-perovskite systems to antiferroelectric ordering, frustrated by structural disorder. Quenching the samples from high temperature increases cation disorder and suppresses the tendency of these systems toward antiferroelectric instability. A correlation is found between the degree of instability and the parameter η=La1−La2−x, where La1 and La2 stand for the site occupancy factors of lanthanum ions in sites La1 and La2, respectively.