Phase transitions and polar order in BaCexTi1-xO3 (x = 0.02 - 0.30) ceramics

The incorporation of homovalent M4+ ions at the Ti site of BaTiO3 with formation of the BaMxTi1-xO3 solid solutions (M = Sn, Zr, Hf, Ce) determines interesting modifications of the phase transitions, polar order and functional properties. In particular, an evolution of the polar order from conventional ferroelectric to diffuse phase transition behavior and to relaxor state is observed with increasing x. Unlike the systems containing Sn and Zr, Ce-doped barium titanate ceramics have not been fully investigated despite the potential applications as lead-free dielectric and piezoelectric materials. Dense (97-99% rel. density) ceramic samples with composition BaCexTi1-xO3 (x = 0.02, 0.05, 0.06, 0.10, 0.12, 0.15, 0.175, 0.20 and 0.30) were prepared by the conventional solidstate route starting from fine precursor powders and sintered at 1450-1500 °C. The average crystal structure at different temperatures in the range 100-400 K has been determined from the Rietveld refinement of high-energy X-ray diffraction data collected at ESRF. The pair distribution function analysis has been used to investigate the local structure and the level of disorder. The phase transitions and the evolution of polar order with composition have been studied using dielectric permittivity measurements at 100 Hz-1 MHz between -150 and 150 °C, variable temperature Raman spectroscopy (from -195 to 165 °C) and differential scanning calorimetry (-100-150 °C). Overall, the data provide a detailed picture of the BaCexTi1-xO3 system and a tentative composition-temperature phase diagram will be proposed. The results indicate that the three transitions (rhombohedral/orthorhombic, orthorhombic/ tetragonal and tetragonal/cubic) meet at a critical point located at x ? 0.10 and T = 115 °C. Relaxor behavior is observed at x >= 0.20. In contrast to the homologous systems BaSnxTi1-xO3 and BaZrxTi1-xO3, a diffuse ferro/para transition is already observed at x = 0.05 as a consequence of the high level of lattice disorder.