High-temperature soft phonon behaviour in PLZT 8/65/35 relaxor ferroelectrics

Polar phonons in infrared spectra of (Pb0.88La0.08 0.04)(Zr0.65Ti0.35)O3 ceramics and thin films exhibit similar behaviour. Each phonon allowed in perfect cubic lattice is split not only below the Burns temperature TB = 620 K, where it can be explained with breaking of cubic symmetry in polar nanoclusters, but also above TB. We explain it with two chemical species at both A and B perovskite sites. The E component of the soft mode remains heavily damped at all temperatures and its frequency near 35 cm-1 is almost temperature-independent. The A1 component of the soft mode remarkably hardens below T* 500 K from 50 to 85 cm-1 and moreover a new mode activates in IR spectra below 300 K at frequencies between A1 and E soft modes. Based on comparison with other relaxors, we conclude that such behaviour is general for all relaxor ferroelectrics with perovskite structure.