Optical Properties and Band Structure of SrTiO3and BaTiO3

The normal-incidence reflectance spectra of SrTi${\mathrm{O}}_3$ and BaTi${\mathrm{O}}_3$ and the optical constants derived by the Kramers-Kronig method are reported. These spectra are very similar to those of rutile. The strongest optical transitions ($A$) occur at about 4.4 eV: They are responsible for the dispersion observed below the fundamental absorption edge (3.2 eV). A tentative interpretation of these spectra in terms of the energy band structure of the materials is offered. The $A$ peak seems due to transitions between $2p$ levels of oxygen and $3s$ levels of titanium at the $X$ point of the Brillouin zone. The splitting of the $A$ peak is assigned to the splitting of the $2p$ levels of the oxygen in the cubic field. Transitions between $2p$ oxygen and $4s$ titanium levels, between $2s$ oxygen and $3d$ titanium levels, and between $2s$ oxygen and $5s$ titanium or $6s$ barium levels are also observed. Plasma oscillations are seen around 21 eV.