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 nm(3) of o-BT-II with z = 4 formula units, yields a density rho = 5.214 g/cm(3). This is a new polymorph in comparison to well-known P4mm tetragonal (t) structure, V = 0.0644 nm(3) or rho = 6.016 g/cm(3) (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 epsilon(r) value increases with doping. Curie transition temperature T-C decreases from 128 degrees C to a minimum value of 43 degrees 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 T-C shift and diffuse nature of the dielectric behavior.