Photoluminescence and evolution of polar order in Eu:BaZrx Ti1-xO3 ceramics

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 [1]. BaZrxTi1-xO3 (BZT) solid solution has the perovskite structure and shows a series of phase transitions and a progressive evolution of ferroelectric order, from long-range order typical of classic ferroelectrics (x = 0-0.15), via a diffuse transition behaviour (x = 0.15-0.25), to short-range order typical of relaxors (x >= 0.25) and finally paraelectric non polar state for neat BaZrO3. 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 [2]. In this framework, we have investigated the photoluminescence (PL) emission of dense BZT ceramics doped with Eu3+ at the Ba site with composition EuyBa1-yZrxTi1-x-y/4O3 (y = 0.01; x = 0, 0.05, 0.15, 0.30, 0.50, 0.70, 1) in the temperature range -100 to 140°C. The ceramics were prepared by the classical solid-state route and sintered at 1450-1550 °C. The dielectric permittivity of the samples was measured from -150 to 150 °C at 100 -1000000 Hz to determine reference values of the phase transition temperatures and the type of polar order. The PL spectra underwent significant shape and intensity variations as a function of both composition and temperature, which can be correlated with the nature (long-range or short-range) of polar order. The observed behavior can be rationalized considering the off-center displacement of Eu3+ ions with respect to the lattice position occupied by barium. The strong variation of the PL spectra induced by temperature change can be exploited to develop ratiometric self-calibrating luminescence thermometers [3] working from room temperature down to liquid nitrogen. Keywords: Perovskites, Ferroelectrics, Luminescence References: [1] H. Kishi, Y. Mizuno, H. Chazono, Jpn. J. Appl. Phys. (2003) 42: 1 [2] K. Binnemans, Coord. Chem. Review (2015) 295:1 [3] C. D. S. Brites et al. Nanoscale (2012) 4: 4799