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High-pressure characterization of the optical and electronic properties of InVO4, InNbO4, and InTaO4
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0001-6930-8415
Universidad de Valencia, Valencia, Spain.
Bhabha Atomic Research Centre, Mumbai, India. Homi Bhabha National Institute, Mumbai, India.
Universidad de La Laguna, La Laguna, Spain.
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2019 (English)In: SN Applied Sciences, ISSN 2523-3963, E-ISSN 2523-3971, Vol. 1, no 5, article id 389Article in journal (Refereed) Published
Abstract [en]

We have studied the electronic properties at ambient pressure and under high pressure of InVO4, InNbO4, and InTaO4 powders, three candidate materials for hydrogen production by means of photocatalytic water splitting using solar energy. A combination of optical absorption and resistivity measurements and band structure calculations have allowed us to determine that these materials are wide band-gap semiconductors with a band-gap energy of 3.62(5), 3.63(5), and 3.79(5) eV for InVO4, InNbO4, and InTaO4, respectively. The last two compounds are indirect band-gap materials, and InVO4 is a direct band-gap material. The pressure dependence of the band-gap energy and the electrical resistivity have been determined too. In the three compounds, the band gap opens under compression until reaching a critical pressure, where a phase transition occurs. The structural transition triggers a band-gap collapse larger than 1.2 eV in the three materials, being the abrupt decrease in the band-gap energy related to an increase in the pentavalent cation coordination number. The phase transitions also cause changes in the electrical resistivity, which can be correlated with changes induced by pressure in the band structure. An explanation to the reported results is provided based upon ab initio calculations. The conclusions attained are of significance for technological applications of the studied oxides.

Place, publisher, year, edition, pages
Springer, 2019. Vol. 1, no 5, article id 389
Keywords [en]
Wolframite, Band gap, optical properties, High pressure, Phase transition, Electronic properties
National Category
Other Physics Topics
Research subject
Experimental Physics
Identifiers
URN: urn:nbn:se:ltu:diva-73617DOI: 10.1007/s42452-019-0406-7ISI: 000473581300007Scopus ID: 2-s2.0-85067022763OAI: oai:DiVA.org:ltu-73617DiVA, id: diva2:1304393
Note

Validerad;2019;Nivå 2;2019-08-16 (johcin)

Available from: 2019-04-12 Created: 2019-04-12 Last updated: 2023-03-28Bibliographically approved
In thesis
1. Physical Properties of Ternary Metal Oxides and Carbon Nanomaterials Under Pressure
Open this publication in new window or tab >>Physical Properties of Ternary Metal Oxides and Carbon Nanomaterials Under Pressure
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Found in nature or synthesized, materials present amazing properties such as superconductivity, super-hardness, lightweight, or high-energy-density, among others. All these properties can be used in our benefit to improve or develop new applications. Although, many of these properties are not noticeable in the ambient conditions of pressure and temperature. Therefore, only when the materials are exposed to extreme conditions of temperature, pressure, radiation, etc., become notable. For those reasons, it is fundamental to understand their properties and how they are affected by different parameters such as the synthesis process, morphology, doping or external parameters (e.g. pressure, temperature).

High-pressure studies have been shown to be an excellent tool for proving and study the robustness of material properties as well as for the synthesis of new materials. Changes as extreme and spectacular as converting oxygen gas into a superconducting metal or the well-known graphite to diamond conversion among others have been made under high-pressure conditions.

Among all the materials, and due to their interesting properties, in this doctoral thesis we have studied four ternary metal oxide semiconductors (InVO4, CrVO4, InNbO4 and InTaO4) and carbon nanostructure materials (single-walled carbon nanotubes (SWCNTs)) at ambient conditions as well as under high-pressure (static or dynamic compression) using different characterization techniques such as X-ray diffraction (XRD), Raman spectroscopy (RS), optical absorption, transmission electron microscopy (TEM), photoluminescence (PL) and electrical measurements.

InVO4, InNbO4 and InTaO4 are wide metal oxide semiconductors having band-gap energy of 3.62(5), 3.63(5) and 3.79(5) eV, respectively, being InVO4 a direct band-gap semiconductor and, InNbO4 and InTaO4 indirect band-gap semiconductors. These compounds undergo, under pressure, to a structural phase transition from orthorhombic, in the case of InVO4, or monoclinic, in the case of InNbO4 and InTaO4, to another monoclinic system. This structural phase transition triggers interesting phenomena due to the modification of the electronic band structure of the compounds. Phenomena observed under compression include bandgap collapse about 1-1.5 eV depending on the compound, band crossing due to the change to the local maximum on top of the valence band and colour change. Also, the electrical resistivity of the materials is affected by this change in the band structure. All these results are discussed based on our theoretical band structure calculations.

On the other hand, doping these compounds below 0.2% using Tb or Yb rare-earth elements, the crystal structure is barely affected as well as their phonon structure, but the band structure does, giving rise optical excitation and emission properties in the visible and near-infrared (NIR) spectral region. From optical reflectivity measurements, the two first direct transitions are reported at 3.7/4.2 eV in InVO4, 4.7/5.3 eV in InNbO4 and 5.6/6.1 eV in InTaO4. All the compounds present self-activated photoemission signals which are discussed in terms of the distorted polyhedral coordination around V, Nb and Ta atoms. Finally, the characteristic emission of Tb atoms in the green region (5D4→7FJ) and the Yb atoms in the NIR region (2F5/2→2F7/2) are analysed and discussed based on our theoretical calculations.

Even though, being a prototype structure of a family of compounds denoted as CrVO4-type materials, there is still scarce information on the behaviour under pressure of the CrVO4 compound. Here, it is also studied CrVO4 having an orthorhombic structure under pressure up to 10 GPa. Crystal structure, phonon band structure, optical and electrical properties are analysed showing a structural phase transition similar to that in InVO4 with an increase in the vanadium atoms coordination from 4 to 6. This phase transition triggers also a band-gap collapse of 1.1 eV, a change in the phonon structure and a sharp decrease in the resistivity of the material. All these results are discussed in terms of our theoretical calculations and comparison with its isostructural partner InVO4.

To conclude, we study the effects of the dynamic pressure of 0.5 Mbar (50 GPa) on SWCNTs which is way beyond the limit of their structural stability in quest of new forms of carbon nanostructures. Thus, no nanotubes survived to this pressure. The recovered material is composed of two types of material which are classified in a multi-layer graphene phase (MLG) with high defect concentration and multi-phase material which dominates the sample. Even the reached conditions during the shock-compression were favourable for the diamond formation, we were unable to find traces of diamond-like carbon in the very inhomogeneous sample. The crystal size of both materials has been estimated at 13 nm for disordered carbon and 30 nm for MLG phase. The dispersion of the Raman modes was also studied using several lasers and the observations were supported by TEM analysis.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2020. p. 120
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Other Materials Engineering Other Physics Topics
Research subject
Experimental Physics
Identifiers
urn:nbn:se:ltu:diva-78581 (URN)978-91-7790-587-5 (ISBN)978-91-7790-588-2 (ISBN)
Public defence
2020-06-18, A109, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2020-04-20 Created: 2020-04-20 Last updated: 2020-08-21Bibliographically approved

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