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The electrical properties of the entropy stabilized oxides: Zr6Nb2O17, Zr6Ta2O17, Hf6Nb2O17and Hf6Ta2O17were characterized. The results and the electrical properties of the products (i.e. ZrO2, HfO2, Nb2O5and Ta2O5) led us to hypothesize the A6B2O17family is a series of mixed ionic-electronic conductors. Conductivity measurements in varying oxygen partial pressure were performed on A6Nb2O17and A6Ta2O17.The results indicate that electrons are involved in conduction in A6Nb2O17while holes play a role in conduction of A6Ta2O17. Between 900 °C–950 °C, the charge transport in the A6B2O17system increases in Ar atmosphere. A combination of DTA/DSC and in situ high temperature X-ray diffraction was performed to identify a potential mechanism for this increase. In-situ high temperature X-ray diffraction in Ar does not show any phase transformation. Based on this, it is hypothesized that a change in the oxygen sub-lattice is the cause for the shift in high temperature conduction above 900 °C–950 °C. This could be:(i)Nb(Ta)4+- oxygen vacancy associate formation/dissociation,(ii)formation of oxygen/oxygen vacancy complexes(iii)ordering/disordering of oxygen vacancies and/or(iv)oxygen-based superstructure commensurate or incommensurate transitions. In-situ high temperature neutron diffraction up to 1050 °C is required to help elucidate the origins of this large increase in conductivity.
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Exploring Cation Selection and Disorder Within Entropy‐Driven A 6 B 2 O 17 ( A = Zr/Hf, B = Nb/Ta) Oxides
ABSTRACT We investigate the local atomic and electronic structure, thermodynamic stability, and defect chemistry ofA6B2O17(A= Zr/Hf,B= Nb/Ta) oxides using first‐principles density functional theory (DFT) calculations. We examine both ordered unit cells as well as fully disordered special quasirandom structures to clearly discern the effects of cation disorder. Structural predictions align closely with previous experimental results and follow established ionic radii trends. The electronic structure is strongly dependent onB‐cation species:A6Ta2O17compositions have ~30% larger band gaps than theirA6Nb2O17counterparts. Defect chemistry is similar for all compositions, with anion vacancies being more energetically favorable than corresponding cation defects. All exploredA6B2O17compositions are enthalpically unstable with respect to theirAO2andB2O5competing oxides and are therefore classified as entropy‐stabilized materials, supporting prior experimental results. The pronounced agreement between our disordered supercell predictions and experimental measurements indicates all exploredA6B2O17compositions contain substantial cation disorder across all 6‐, 7‐, and 8‐coordinated sites. Our findings collectively provide a fundamental understanding of theA6B2O17material family through DFT calculations, establishing a framework for future compositional tuning to engineer targeted material properties.
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- Award ID(s):
- 2011839
- PAR ID:
- 10683391
- Publisher / Repository:
- The American Ceramic Society
- Date Published:
- Journal Name:
- Journal of the American Ceramic Society
- Volume:
- 109
- Issue:
- 4
- ISSN:
- 0002-7820
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1111/jace.70652
