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The origin of low bandgap and ferroelectricity of a co-doped BaTiO3
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.
Department of Solid State and Structural Chemistry Unit, Indian Institute of Science - Bengaluru 560012, India.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
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2018 (English)In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 124, no 2, article id 27005Article in journal (Refereed) Published
Abstract [en]

We recently demonstrated the lowest bandgap bulk ferroelectric material, BaTi1-x(Mn1/2Nb1/2)xO3, a promising candidate material for visible light absorption in opto- electronic devices. Using a combination of x-ray spectroscopies and density functional theory (DFT) calculations, we here elucidate this compound’s electronic structure and the modifications induced by Mn doping. In particular, we are able to rationalize how this compound retains its ferroelectricity even through a significant reduction of the optical gap upon Mn doping. The local electronic structure and atomic coordination are investigated using x-ray absorption at the Ti K, Mn K, and O K edges, which suggests only small distortions to the parent tetragonal ferroelectric system, BaTiO3, thereby providing a clue to the substantial retention of ferroelectricity in spite of doping. Features at the Ti K edge, which are sensitive to local symmetry and an indication of Ti off-centering within the Ti-O6 octahedra, show modest changes with doping and strongly corroborates with our measured polarization values. Resonant photoelectron spectroscopy results suggest the origin of the reduction of the bandgap in terms of newly created Mn d bands that hybridize with O 2p states. X-ray absorption spectra at the O K-edge provide evidence for new states below the conduction band of the parent compound, illustrating additional contributions facilitating bandgap reduction.

Place, publisher, year, edition, pages
2018. Vol. 124, no 2, article id 27005
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-364369DOI: 10.1209/0295-5075/124/27005ISI: 000450630500002OAI: oai:DiVA.org:uu-364369DiVA, id: diva2:1258790
Funder
Swedish Foundation for Strategic Research , 15-0130Knut and Alice Wallenberg Foundation, 2012.0031Swedish Energy Agency, P43549-1Swedish Energy Agency, P43294-1Swedish Research Council, 2014-7019Swedish Research Council, 2016-4524
Note

Corection in: EPL, Volume: 124, Issue: 6, Article Number: 69901, DOI: 10.1209/0295-5075/124/69901

Available from: 2018-10-25 Created: 2018-10-25 Last updated: 2019-02-04Bibliographically approved
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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Phuyal, DibyaMukherjee, SohamJana, SomnathRensmo, HåkanButorin, Sergei M.Karis, Olof

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