X-ray spectroscopic study of the electronic structure of the high-dielectric-constant materialCaCu3Ti4O12

Abstract

The element-specific valence- and conduction-band densities of states for the high-dielectric-material $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ have been measured using soft x-ray emission and absorption spectroscopies. Ti ${L}_{\ensuremath{\alpha},\ensuremath{\beta}}$, Cu ${L}_{\ensuremath{\alpha},\ensuremath{\beta}}$, and O $K$ soft x-ray emission spectra of $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ were measured with monochromatic photon excitation on selected energies above the Ti and Cu ${L}_{2,3}$ and O $K$ absorption edges, respectively. X-ray absorption spectra were recorded at the same edges. The electronic structure was also calculated using density functional theory employing the full-potential linearized augmented plane-wave method. Excellent agreement is seen between the results of these calculations and the measured x-ray emission and absorption spectra. This agreement is particularly good at the O $K$ edge where the resonant behavior of the x-ray emission spectrum can be attributed directly to $\ensuremath{\sigma}$- and $\ensuremath{\pi}$-state emission from valence-band O $2p$ states when in resonance with ${\ensuremath{\pi}}^{*}$ and ${\ensuremath{\sigma}}^{*}$ conduction-band O $2p$ states. Resonant inelastic x-ray scattering is observed at the Ti ${L}_{2,3}$ absorption edge and is compared to previous studies of Ti containing perovskite compounds. The role of Cu $3d$ states in determining the band gap of this material is discussed.