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-delta 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-delta 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.