Band-gap nonlinearity in perovskite structured solid solutions

Compositional effects on optical band-gap energy using end members of ABO₃ perovskites have been investigated through an optical absorption with a UV-spectroscopy. Three examples are selected, namely, BaTiO₃ - CaTiO₃, BaTiO₃ - BaZrO₃, and SrTiO ₃ - BaZrO₃ solid solutions. To understand the role of high temperature phase equilibria on the band-gap compositional trends, structural and microscopy data were determined. In simple systems such as Si-Ge, the nonlinear variations in band gap with composition is usually associated with the effect of the local lattice relaxations and provides a parabolic dependence, often referred to as the "bowing" phenomena. In the case of perovskite solutions, the cases are more complex, and a modified Vegard's law is introduced to account for the trends. This has to be considered in relation to high temperature phase formation where incomplete solid solutions and two-phase regions exist. In addition to high temperature phases, low temperature displacive phase transitions and complex nonstoichiometry also perturb the band-gap variation in perovskite oxide materials.