Dielectric properties of 3d transition metal substituted BaTiO3 ceramics containing the hexagonal phase formation

The role of transition metals of the 3d series from V (Z = 23) to Zn (Z = 30) is investigated in modifying the crystallographic phase contents, microstructure and the dielectric properties of BaTiO3 ceramics containing <= 10 at% substituents. All the transition metals brought about the phase conversion to hexagonal BaTiO3 and the hexagonality is found to depend on Ba/Ti ratio as well as the processing conditions including the sintering temperature and the post sinter annealing. The epsilon(r)-T characteristics are modified with increasing hexagonality by way of the tremendous decrease in dielectric constant with broad and diminished epsilon(max) for the mixed phase ceramics giving way to flat epsilon(r)-T curves for totally hexagonal specimens. Doping with > 1% Zn(2+)or >= 5% Mg2+ also render the ceramics completely hexagonal indicating that the crystal field effects of the 3d orbital electrons are not the cause for the conversion to hexagonal phase. Electron paramagnetic resonance (EPR) spectra of Mn-doped ceramics reveal the prevalence of defect complexes involving oxygen vacancies and different valence states of Mn occupying the Ti-sites within the corner-sharing as well as face-sharing octahedra present in hexagonal BaTiO3. EPR results also indicate defect complexes involving electron localization at Ti-sites and oxygen vacancies around the face-sharing octahedra. On annealing the ceramics in lower oxygen partial pressures at elevated temperatures leads to the reversion to corner shared (Ti3+-V-O) defect complex accompanied by the conversion to cubic/tetragonal phase. The alterations in the oxygen vacancy-metal impurity defect complexes and the modifications in the oxygen close packing are the cause for the prevalence of hexagonal BaTiO3 at room temperature.