Viscoelastic sigmoid anomalies in BaZrO3-BaTiO3 near phase transformations due to negative stiffness heterogeneity

BaZrO3-BaTiO3 ceramics exhibit a shift in transformation temperatures as revealed by dielectric and viscoelastic spectroscopy; a phase diagram has been established. Sigmoid anomalies in Poisson's ratio and bulk modulus during the ferroelastic transitions were observed in doped materials, which are not predicted by standard theories for phase transformations. Hashin-Shtrikman composite model with negative stiffness heterogeneity can well explain this phenomenon. Negative stiffness heterogeneity is considered to be caused by the strained BaTiO3 unit cells in the vicinity of BaZrO3-rich zones under the perturbation of lattice reconstruction.