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Electronic structure of the charge disproportionate and metallicity in LaFe1-xMoxO3 studied by resonant x-ray spectroscopies
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.ORCID iD: 0000-0003-0351-3138
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

A metal-insulator transition and its intriguing properties were recently reported based on doping of the antiferromagnetic insulator LaFeO3. Through doping at the B site by Mo to the insulating perovskite LaFeO3 exhibits a coexistence of an antiferromagnetic ordering with room-temperature metallicity at higher doping levels, an unusual ground state for an oxide perovskite. Replacing Fe by Mo in the Fe-O6 octahedra enhances the separation of the two 2 Fe-O bonds relative to the orthorhombic LaFeO3, largely retaining the centrosymmetric character of the Fe sites, as confirmed through extended x-ray absorption fine structure. Mo ions appear to be homogeneously doped, with average valency of both metal sites monotonically decreasing with increasing Mo concentration. Resonant photoemission was recorded for both Fe and Mo to obtain element specific spectral information on the projected 3d and 4d partial density of states. The data shows gradual increase of spectral weight at the Fermi level that is attributed to Mo d states, while the Fe d states primarily reside at higher binding energy in the valence band, with significant hybridization with O 2p states. The RIXS spectra show strong electron correlation effects from d-d inelastic features and broad charge-transfer excitations for x = 0.2 where the compound becomes metallic. This is in contrast to the insulating parent LaFeO3 compound and insulating samples with lower Mo content. The coexistence of both bound and continuum excitations observed in the RIXS spectra provides strong evidence for charge disproportionation with features that are linked to the disproportionate Fe sites.

National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-364367OAI: oai:DiVA.org:uu-364367DiVA, id: diva2:1258787
Funder
Knut and Alice Wallenberg Foundation, 2012.0031Swedish Energy Agency, P45349-1Swedish Research Council, 2014-6019Carl Tryggers foundation , CTS-17:376Available from: 2018-10-25 Created: 2018-10-25 Last updated: 2018-10-26
In thesis
1. An X-ray Spectroscopic Study of Perovskites Oxides and Halides for Emerging Devices
Open this publication in new window or tab >>An X-ray Spectroscopic Study of Perovskites Oxides and Halides for Emerging Devices
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis investigates the electronic structures on several perovskite oxide and halide materials with a focus on light harvesting applications. The systematic study of the electronic properties of the transition metal oxides and post-transition metal halides is a key point if one is to understand their properties. The element and site selective nature of several x-ray based spectroscopic techniques are given special emphasis in order to obtain a complete picture of the electronic properties of the compounds in question. Much of the experimental studies are accompanied by ab initio calculations that corroborate with our experimental results.

In the oxide portion of this work, a new class of metallic oxides based on doping of an antiferromagnetic LaFeO3 was synthesized and systematically studied with x-ray absorption, x-ray emission, and photoemission spectroscopies. The compound’s electronic structure is complex, having itinerant as well as localized components that give rise to a unique physical state where antiferromagnetism, metallicity and charge-disproportionation coexist. Our resonant photoemission results establish that the Fe states in both magnetically ordered oxides show insulting properties, while the Mo states provide an itinerant band crossing the Fermi level. An excitation energy-dependent RIXS investigation on LaFe1-xMoxO3 and the double perovskite Sr2FeMoO6 revealed a double peak structure located in proximity to the elastic peak that is identified to purely d-d excitations, attributed to the strongly correlated nature of these transition metal compounds.

The growth of high-quality thin film ferroelectric based on BaTiO3 grown epitaxially by means of pulsed laser deposition were investigated. We systematically reduce the band gap of the ferroelectric thin film while retaining its polarization at ambient conditions in spite of the aliovalent doping. The electronic structure is studied by several x-ray techniques that show how the ferroelectricity persists as well as the effective reduction of the band gap through hybridized states.

In the post-transition metal halides, the valence and conduction bands were mapped using x-ray absorption, emission, and photoemission spectroscopies. The spectroscopic results identify the constituent states that form the valence band as well as the band energy positions, which is an imperative parameter in optoelectronic devices. In addition, x-ray based spectroscopy was used to demonstrate the stereochemical activity of lone-pair states (5s2 and 6s2) for several different halide compounds and their influence on the chemical, structural, and electronic properties of the material. Nanostructured halide perovskites are also explored. The position of iodine p states and valence band states in reduced dimensional lead-based compounds were examined, as their states are found to be confined in one crystallographic direction in contrast to their three-dimensional counterpart. This information highlights the interesting material properties and their use in current third generation solar cell research.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 84
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1740
Keywords
perovskite oxides, halide perovskites, x-ray spectroscopy, electronic structure
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-364407 (URN)978-91-513-0493-9 (ISBN)
Public defence
2018-12-14, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2018-11-21 Created: 2018-10-26 Last updated: 2018-11-30

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