Structure-property relationships in complex oxides: lattice distortions, excess charges, and small polarons
Date
2022
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Journal ISSN
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Publisher
University of Delaware
Abstract
Metal oxides form a large family of solids with diverse crystal structures and properties, that includes semiconductor, insulator, metal, superconductors, ferroelectric, magnetic, and multiferroic materials. They have been explored for a variety of technological applications, from photoelectrochemical energy conversion, transparent conductors, gate dielectrics in field-effect transistors, and catalysis, fuel cells, batteries, to name a few. In many of these applications, electronic and ionic transport, charge localization, ferromagnetic, and ferroelectric properties are intimately related to the crystal structure, and small modifications in the lattice can cause large changes in the physical properties. Using state of the art first-principles calculations based on density functional theory (DFT) and hybrid functionals, we investigate how this relationship can explain some puzzling recent observations and give rise to novel phenomena. Focusing on selected prototypical metal oxides, that include multiferroics, photocatalysts, Mott insulators, and semiconductors, we studied how details in the crystal structure affect the electronic properties, the effects of charge carrier on structural properties, charge localization in the form of small polarons, and carried out a systematic analysis of the performance of the most used functionals in describing band gaps, ionization potentials, and electron affinities for representative metal oxides that include high-k dielectrics, semiconductors, insulators, Mott insulators, ferroelectrics and magnetic materials. ☐ We explore structure-property relationships in multiferroic (ferroelectric, antiferromagnetic) BiFeO3, a material that has been proposed for memory devices. We investigate the dependence of the electronic structure on the crystal lattice distortions, disentangling the effects of the ferroelectric ionic displacements and the antiferrodistortive octahedral rotations on the band gap and the band-edge positions. We find that the gap varies linearly with the ferroelectric ionic displacements, but nonlinearly with the octahedral rotations around the pseudocubic [111]c axis, and this is explained in terms of the different interactions between Bi 6s, 6p, Fe 3d, and O 2p bands. In the case of BiVO4, a prototypical oxide for photoelectrochemical water splitting, we attempt to resolve the problem of spontaneous transformation of the photoelectrochemically active monoclinic phase to a tetragonal phase in theoretical description. We investigate how excess electrons in this material affects its phase stability, by analyzing the unit-cell structure, local bonding, and band structure of BiVO4 with different concentrations of conduction-band electrons. We find that as the concentration of excess electrons increases, the tetragonal phase spontaneously transforms into the monoclinic phase, suggesting a crucial role of doping in the structure and, thus, the photoelectrochemical performance of BiVO4. For the rare-earth titanate YTiO3, a typical Mott insulator in the perovskite structure, and that can be used in oxide heterostructures featuring two-dimensional electron gases, we investigate the formation, self-trapping energy, migration, and optical excitation of small hole polarons that can explain the observed thermally activated electrical conductivity in bulk and thin films YTiO3. Additionally, we propose mechanisms involving small hole polarons to explain the observed optical absorption spectra at energies much lower than the predicted Mott-Hubbard gap. We also discuss the appearance of the Dy-f related peaks in the x-ray photoelectron spectroscopy (XPS) measurements in YTiO3 thin films grown on DyScO3 substrates by molecular beam epitaxy. ☐ Finally, we analyze the performance of different functionals in describing the electronic structure of several metal oxides, focusing on band gap, ionization potential (IP), and electron affinity (EA). We employ methods that go beyond the semi(local) approximations in density functional theory to inspect how ionization potentials and electron affinities are corrected, discussing in particular the performance of the meta-GGA SCAN, DFT+U, and hybrid functionals in the description of some representative semiconductors, band insulators, and Mott insulators, by comparing with available experimental data. We find that with a careful choice of U, DFT+U does correct band gaps in most cases, however, it often leads to wrong descriptions of IP and EA; whereas the screened hybrid functional of Heyd-Scuseria-Ernzerhof with appropriate mixing parameter, leads to accurate description of IP and EA in most cases. Our work indicates that interpretations of widely used high-throughput screening of materials for catalysis, photovoltaics, or batteries (where IP and EA, and band gaps are crucial parameters) based on the computationally inexpensive DFT+U method should be taken with great care.
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Keywords
Metal oxides, Crystal, Rare earth materials