Influence of the calcium excess in the structural and spectroscopic properties of the complex perovskite Ba3CaNb2O9

The effects of the change in the Ca/Nb ratio in the structure and spectroscopic properties of the solid solution Ba3Ca1+xNb2-xO9-δ with x = 0–0.25 are investigated. The solid solution undergoes a transition from an hexagonal phase (2:1 order) to a cubic phase (1:1 order) with increasing calcium content as indicated by X-Ray diffraction and Raman. The calcium excess matches with a gradual increase in blue colour due to a reduction at the high temperatures of sintering. Optical absorption experiments showed that there are different absorption bands in the region 0.45–4.5 eV corresponding tentatively to OH vibrations, polarons and monovalent oxygen vacancies. The series properties change gradually with calcium excess with the exception of Ba3Ca1.18Nb1.82O9-δ that showed a slight change attributable to the presence of protons in the lattice. These changes can be used to quantify protons in complex perovskites.