X-ray spectroscopic study of the electronic structure of the high-dielectric-constant material Ca Cu3 Ti4 O12

The element-specific valence- and conduction-band densities of states for the high-dielectric-material Ca Cu3 Ti4 O12 have been measured using soft x-ray emission and absorption spectroscopies. Ti Lα,β, Cu Lα,β, and O K soft x-ray emission spectra of Ca Cu3 Ti4 O12 were measured with monochromatic photon excitation on selected energies above the Ti and Cu L2,3 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 σ - and π -state emission from valence-band O 2p states when in resonance with π* and σ* conduction-band O 2p states. Resonant inelastic x-ray scattering is observed at the Ti L2,3 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.