Electron-spectroscopy study of the semiconductor-metal transition in ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CoO}}_3$

A study of the semiconductor-metal transition in the series ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CoO}}_3$, x=0.0–0.4, using electron-spectroscopy techniques is presented. The results show that the ground state of ${\mathrm{LaCoO}}_3$ has strongly mixed character. From the experiments we estimate the on-site Coulomb correlation energies ${\mathit{U}}_{\mathit{d}\mathit{d}}$ and ${\mathit{U}}_{\mathit{p}\mathit{p}}$ to be 3.4 and 6.7 eV, respectively, while the (${\mathit{pd}}_{\mathrm{\sigma }}$) interaction strength is 2.2 eV. A comparison of the experiment with the results of a model many-body cluster calculation for the Co 2p spectra indicates a low-spin state of Co in ${\mathrm{LaCoO}}_3$. The band gap of ${\mathrm{LaCoO}}_3$ is estimated to be about 0.6 eV from ultraviolet photoemission spectroscopy and bremsstrahlung isochromat spectroscopy measurements. The semiconductor-metal transition results from overlap of doped hole states with the valence band for x≥0.2.