Raman scattering in theRTiO3family of Mott-Hubbard insulators

Raman-scattering measurements have been carried out for crystals of the ${\mathrm{RTiO}}_3$ (R=La,Ce,Pr,Nd,Sm,Gd) system whose members are Mott-Hubbard insulators. ${\mathrm{RTiO}}_3$ has an orthorhombically distorted perovskite unit cell. The distortion increases systematically from ${\mathrm{LaTiO}}_3$ to ${\mathrm{GdTiO}}_3$ and is accompanied by changes in electronic structure (decreasing W/U ratio). As a consequence of the changing electronic properties, the Raman spectrum shows an interesting evolution of both the phonon features and the electronic continuum. Most notable are (1) a redistribution in the spectral shape of the electronic background, (2) a systematic change in line shape, and a dramatic increase in the center frequency of one of the phonon modes from 287 ${\mathrm{cm}}^{\mathrm{-}1}$ in ${\mathrm{LaTiO}}_3$ to 385 ${\mathrm{cm}}^{\mathrm{-}1}$ in ${\mathrm{GdTiO}}_3$, and (3) the observation of resonance effects in the most insulating members of the series. The appearance of a free-carrier component in the electronic-scattering background, which seems to be related to systematic self-energy effects of the phonon near 300 ${\mathrm{cm}}^{\mathrm{-}1}$, is unexpected. It is likely the result of increased doping due to a greater facility for rare-earth vacancies to form in large ${\mathrm{R}}^{3+}$ ionic radius members of the series. A systematic increase in the continuum scattering rate is also observed and indicates that the free carriers are not scattering off rare-earth vacancies but rather that the scattering mechanism originates from changes in electronic structure.