The optical properties of R titanium oxides perovskites (R = lanthanum, cerium, praesodymium, neodymium, samarium, gadolinium)

Abstract

<p>Reflectance spectroscopy (50-40000 cm⁻¹) has been used to study the rare earth titanium (III) oxides (RTiO\sb3$, R = La, Ce, Pr, Nd, Sm, Gd). A mid-infrared absorption has been observed in all members of the series. With decreasing rare earth radius (and decreasing Ti-O-Ti bond angle) the band shifts to higher frequencies. A tight-binding calculation of the effect of the Ti-O-Ti bond angle on the conduction band width supports the picture in which the mid-infrared band is due to transition from the lower Hubbard to the upper Hubbard band. Because this intrinsic absorption is at lower energy than transitions corresponding to O(2p)-Ti(3d) charge transfer (≈ 4 eV) the rare earth titanites are classified as Mott-Hubbard insulators. Systematic variations in the infrared active phonon spectra were observed and related to a tetragonal distortion of the TiO₆ octahedron in NdTiO₃, SmTiO₃ and GdTiO₃. In a second study, the reflectance of four well characterized (x-ray, neutron activation, thermogravimetric, dc resistivity, magnetization) doped LaTiO₃ samples was measured. Although the starting materials contained equimolar amounts of La and Ti, the neutron activation results and structural considerations indicate the presence of La vacancies. There is a transfer of oscillator strength from the mid-infrared band to lower frequencies in highly doped samples but transitions placing two electrons on one site (from the lower to the upper Hubbard band) retain considerable oscillator strength even in a sample where approximately one-fifth of the sites are empty.</p>