Anelastic anomalies and negative Poisson's ratio in tetragonal BaTiO3 ceramics

Anelastic anomalies (sharp variations in modulus and damping with temperature) were observed in tetragonal BaTiO3 via broadband viscoelastic spectroscopy after aging at 50 degrees C for 15 h. The effect was most pronounced under electrical short circuit condition, at low frequency and under small excitation strain (10(-6)). Softening in bulk modulus and negative Poisson's ratio were observed near 60 degrees C. Effects are attributed to an oxygen vacancy mechanism. A relaxational model cannot account for sharp response at smaller strains. Heterogeneity of negative stiffness is considered as a cause.