Experimental and Theoretical Study of the Electronic Structures of Lanthanide Indium Perovskites LnInO.

Ternary lanthanide indium oxides LnInO (Ln = La, Pr, Nd, Sm) were synthesized by high-temperature solid-state reaction and characterized by X-ray powder diffraction. Rietveld refinement of the powder patterns showed the LnInO materials to be orthorhombic perovskites belonging to the space group , based on almost-regular InO octahedra and highly distorted LnO polyhedra. Experimental structural data were compared with results from density functional theory (DFT) calculations employing a hybrid Hamiltonian. Valence region X-ray photoelectron and K-shell X-ray emission and absorption spectra of the LnInO compounds were simulated with the aid of the DFT calculations. Photoionization of lanthanide 4f orbitals gives rise to a complex final-state multiplet structure in the valence region for the 4f compounds PrInO, NdInO, and SmInO, and the overall photoemission spectral profiles were shown to be a superposition of final-state 4f terms onto the cross-section weighted partial densities of states from the other orbitals. The occupied 4f states are stabilized in moving across the series Pr-Nd-Sm. Band gaps were measured using diffuse reflectance spectroscopy. These results demonstrated that the band gap of LaInO is 4.32 eV, in agreement with DFT calculations. This is significantly larger than a band gap of 2.2 eV first proposed in 1967 and based on the idea that In 4d states lie above the top of the O 2p valence band. However, both DFT and X-ray spectroscopy show that In 4d is a shallow core level located well below the bottom of the valence band. Band gaps greater than 4 eV were observed for NdInO and SmInO, but a lower gap of 3.6 eV for PrInO was shown to arise from the occupied Pr 4f states lying above the main O 2p valence band.