The midinfrared absorption in RTiO3 perovskites (R = La, Ce, Pr, Nd, Sm, Gd) The Hubbard gap?

We have measured the room temperature reflectance of the rare earth titanites (RTiO3, R=La, Ce, Pr, Nd, Sm and Gd) between 50 and 40000 cm−1. A midinfrared absorption exists in all members of the series which systematically shifts to higher frequencies as the Ti-O-Ti bond angle decreases. The absorption may be related to the Hubbard gap. Slightly oxidized samples of LaTiO3 exhibit a change from insulating to metallic resistivity near the Néel temperature. The reflectance of one such sample has been measured between 20 and 300 K and the Kramers-Kronig derived optical conductivity suggests that the onset of the midinfrared absorption tends toward zero frequency in the metallic phase.