Synthesis and characterisation of Ga-doped hexagonal BaTiO3

The hexagonal polymorph of BaTiO₃ (P6₃/mmc) has been stablished at room temperature by partial replacement of Ti by Ga, where BaTi_1-yGa_yO_3-y/2 and 0.06≤y≤0.125 for samples prepared at 1300°C. The unit cell expands with increasing y and Rietveld Refinement of Neutron diffraction data shows that oxygen vacancies occur only in the hexagonal close packed layers between the face-sharing Ti₂O₉ dimers. Exaggerated grain growth (>100 μm) occurs for ceramics processed at ≥1400°C and/or for sintering periods ≥2 hours. Electrical measurements show the materials to be electrically insulating with room temperature permittivity values of ~70–80. Dense ceramics (94–97% of the theoretical X-ray density) resonate at microwave frequencies with Q.f values of ~4000–8000 at ~5.5 GHz.