Photoluminescence-structure correlations in ferroelectric Eu:Ba(Ti,Zr)O3 ceramics: study of phase transitions and polar order

BaTiO3-based ferroelectric ceramics show interesting properties such as high dielectric constant, low losses, high hydrostatic piezoelectric coefficient and PTCR effect. For these reasons, barium titanate is widely used in the electronic industry for manufacturing miniaturized multilayer ceramic capacitors, underwater transducers and self-regulating thermistors. BaZrxTi1-xO3 (BZT) solid solution shows a series of phase transitions and a progressive evolution of ferroelectric order, from long-range order typical of classic ferroelectrics (x = 0) to short-range order typical of relaxors (x >= 0.25), which consequently affects the functional properties. This material can be considered as a model system for understanding the composition-property correlations in ferroelectric ceramics. The trivalent europium ion (Eu3+) is well known for its strong luminescence in the red spectral region. Even very small variations in the coordination sphere of europium ions induce major changes in the emission spectrum. Thanks to these features, Eu3+ is a unique and powerful local structural probe. In this framework, we used photoluminescence (PL) as a tool to investigate the phase transitions and the ferroelectric order in BZT using Eu3+ as active centre. Dense ceramics (relative density: 96-99%) with composition EuyBa1-yZrxTi1-x-y/4O3 (y = 0.01, x = 0, 0.05, 0.15, 0.30, 0.50, 0.70, 1) were prepared by the classical solid-state route and sintered at 1450-1500 °C. The dielectric permittivity of the samples was measured from -150 to 150 °C and at 102-106 Hz to determine reference values of the phase transition temperatures. The study of PL spectra as a function of temperature, in the interval -100 to 140 °C, revealed strong shape and intensity variations of the emission bands in the proximity of phase transitions as well as a significant dependence of the band intensity on x in the composition region (x >= 0.30) corresponding to the relaxor behaviour.