Doping dependence of the optical properties of low-dimensional perovskite-related La1-yCayTiO3.4±δ

The doping dependence of the optical properties of La1-yCayTiO3.4 +/-delta crystals was studied by polarization-dependent infrared reflectivity measurements at room temperature. For y < 0.2 the optical conductivity spectra reveal a quasi-one-dimensional conducting character, and the phonon spectra suggest small changes in the crystal structure and effective charge induced by the substitution of trivalent lanthanum with divalent calcium. The pronounced mid-infrared absorption band observed along the conducting direction shifts to lower frequencies with increasing electron concentration. The possible origin of the band is discussed in terms of polaronic excitations, Mott-Hubbard excitations, phonon-assisted Ti t(2g) d-d transitions, and charge-transfer (2p-3d) transitions. Among these interpretations the polaron model is most likely. La1-yCayTiO3.4 +/-delta with y = 0.2 and d = 0 exhibits an insulating character and a less anisotropic optical response. The observed differences in the electronic and vibrational excitations in La0.8Ca0.2TiO3.40 compared to the other studied titanates could be due to local changes of the crystal structure.