Experimental and Theoretical Study of the Electronic Structures of Lanthanide Indium Perovskites LnInO₃

Ternary lanthanide indium oxides LnInO₃ (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 LnInO₃ materials to be orthorhombic perovskites belonging to the space group Pnma, based on almost-regular InO₆ octahedra and highly distorted LnO₁₂ 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 LnInO₃ 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 4fⁿ compounds PrInO₃, NdInO₃, and SmInO₃, and the overall photoemission spectral profiles were shown to be a superposition of final-state 4f^n–1 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 LaInO₃ 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 NdInO₃ and SmInO₃, but a lower gap of 3.6 eV for PrInO₃ was shown to arise from the occupied Pr 4f states lying above the main O 2p valence band.