Influence of Cobalt Ion Doping on Tetragonal–Orthorhombic Polymorphic Transformation and Dielectric Behavior of Barium Titanate Nanoparticles

Cobalt (Co) ion-doped barium titanate (BaTiO3, BT) nanoparticles were synthesized through the sol–gel route using polyvinyl alcohol. The concentration of Co2+ varies from 0 to 1.6 mol% in the specimens. Grain size (D) ranges from 21 to 29 nm. There exist a new phase of Pnma orthorhombic crystal structure (o-BT-II) with lattice parameters a=0.6393 nm, b=0.5268 nm, and c=0.8824 nm in addition to P4mm tetragonal phase in the specimens. The lattice volume V=0.2971 nm3 of o-BT-II with z=4 formula units, yields a density ρ=5.214 g/cm3. This is a new polymorph in comparison to well-known P4mm tetragonal (t) structure, V=0.0644 nm3 or ρ=6.016 g/cm3 (z=1). In 0.6 mol% Co2+-doped specimen o-BT-II phase exists with maximum volume fraction of 64. Measurement of dielectric properties showed that the room temperature ɛr value increases with doping. Curie transition temperature TC decreases from 128°C to a minimum value of 43°C in the specimens doped with 0.6 mol% dopant. The presence of space charge polarization in system is clear from dielectric-frequency dispersion study. The coexistence of tetragonal and orthorhombic (o-BT-II) phase results an unusual TC shift and diffuse nature of the dielectric behavior.