Title: Phase superposition as origin of enhanced functional properties in BaTiO3 - based ceramics

Abstract

The role of polymorph superpositions around room temperature in BaTiO3 ceramics with grain size (GS) between 75÷2250 nm was investigated to check the GS-dependence of their structural and functional properties. Superposition of orthorhombic-tetragonal or of more polymorphs with variable amounts around room temperature are affected by size reduction and plays an important role, mostly on the properties measured after dc-poling (pyro/piezoelectric properties). Permittivity vs. dc field dependences shows field-induced structural transformations (slope modification) at dc fields 10 kV/cm, which were confirmed by XRD analysis realized in remanence on poled ceramics. The phase superposition around room temperature is a tool which can be exploited to enhance functional properties and is responsible with high permittivity, piezoelectric and pyroelectric constants in BaTiO3-based ferroelectric ceramics. In porous BaTiO3 ceramics with phase coexistence, low density favors the tetragonal state while high density promotes the stability of lower symmetry phases (orthorhombic, rhombohedral or both). For doped-BaTiO3 ceramics, the high polarization and piezo/pyroelectric constants are reached when orthorhombic-tetragonal superposition is obtained around room temperature (for example, in 5% Sn additions). This is explained on the basis of thermodynamic models. A Landau-based calculation indicates the possibility of metastable polymorph states coexistence around room temperature. The phase composition may be altered by dc fields (field-induced structural transformations) and by the application of strain-stress fields produced by substitutions, structural point defects or dislocations. In conclusion, even in a classical well-known ferroelectric material as BaTiO3, the coexistence of polymorphs may be induced in specific conditions and it can be used to enhance the properties.