Structure-property correlations and origin of relaxor behaviour in BaCexTi1-xO3

Macroscopic properties measurements, such as dielectric permittivity and ferroelectric hysteresis, differential scanning calorimetry, and average structure information are combined with complementary techniques sensitive to the local structure, i.e. Pair Distribution Function (PDF) and Raman spectroscopy, to gain comprehensive insight into the structure-property relationships and origin of relaxor behaviour in BaCexTi1-xO3 ceramics over a broad composition (x = 0.02-0.30) and temperature (100-450 K) range. The resulting phase diagram displays sequential phase transitions with a tricritical point (TCP) at x = 0.09 and a ferroelectric to relaxor crossover (FRC) at x ? 0.20. In contrast, the local structure is rhombohedral irrespective of x and the PDF reveals the existence of a high level of disorder and significant local strains determined by the ionic size mismatch (Ce4+: 0.87 Å, Ti4+: 0.605 Å). The diffuse character of the phase transitions observed when x >= 0.05 is most likely originated by these deformations. Parallel of BaMxTi1-xO3 phase diagrams (M = Sn, Hf, Zr, Ce) shows that the compositions corresponding to TCP and FRC are nearly independent of M. This suggests that, irrespective of the ionic radius of M4+, in homovalent-substituted BaTiO3 a critical number of Ti-O-Ti bonds has to be broken before a new "state" is established, whereas local electric and strain fields seem to have a marginal effect.