Crystal and defect chemistry influences on band gap trends in alkaline earth perovskites

Abstract

A number of perovskites with A-site alkaline earth chemistries being Ca, Sr, and Ba, and tetravalent cations including Ce, Zr, and Ti are measured for optical band gap and found to vary systematically with tolerance factor and lattice volume within limits defined by the chemistry of the octahedral site. This paper also focuses on the BaTiO3 system, considering equilibrated nonstoichiometries, and determines the changes in band gap with respect to Ba∕Ti ratios. It was found that the optical band gap changes in the solid solution regime and is invariant in the second phase regions, as would be expected. In the cases of Ba∕Ti<1.0, the variation in band gap scales with lattice volume, but in the Ba∕Ti>1.0 stoichiometries, there is a distinct Urbach tail and the trend with lattice volume no longer holds. It is inferred that the VTi⁗−2VO•• partial Schottky complex controls the band gap trend with Ba-rich nonstoichiometries.