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

P Hartley,R G Egdell,D O Scanlon,L F J Piper,A Regoutz,D J Morgan,K H L Zhang,B A D Williamson,M V Hohmann

doi:10.1021/acs.jpcc.0c11592

Abstract

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 4fn–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.

Highlights

There is a growing interest in lanthanide indium perovskitesLnInO3 oxygen related to their applications as phosphor materials,[1−3] ion conductors,[4−9] and photocatalysts.[10]
Band gaps were measured using diffuse reflectance spectroscopy. These results demonstrated that the band gap of LaInO3 is 4.32 eV, in agreement with density functional theory (DFT) calculations
It has been shown that a 2-dimensional electron gas may develop at the LaInO3/BaSnO3 interface,[12−16] similar to that found for LaAlO3/SrTiO3.17 By analogy with BaSnO3, n-type doped LaInO3 has potential as a transparent conducting oxide; donor doping on La site could in principle lead to a material where the donor centers are separated spatially from the In atoms contributing to the conduction band, suppressing ionized impurity scattering.[18]

Summary

Introduction

There is a growing interest in lanthanide indium perovskitesLnInO3 oxygen related to their applications as phosphor materials,[1−3] ion conductors,[4−9] and photocatalysts.[10]. There is a growing interest in lanthanide indium perovskites. LaInO3 itself adopts a perovskite structure based on an array of corner-shared InO6 octahedra, with the larger La3+ ions surrounded by 8 octahedra in a 12-coordinate site. A reduction in symmetry from the cubic perovskite structure to give an orthorhombic phase belonging to the Pnma space group arises from tilting of the InO6 octahedra to accommodate the deviation in the tolerance t factor defined by t = (rLa + rO)/ √2(rIn + rO) from its ideal value of 1 here rE refers to ionic radii of the elements. Based on the ionic radii tabulated by Shannon for 6-coordinate In3+, 12-coordinate Ln3+, and 2-

Methods

Results

Conclusion