Structure property relationships in the ATi2O4 (A=Na, Ca) family of reduced titanates |
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Authors: | Margret J. Geselbracht Ann S. Erickson John E. Greedan Matthew W. Stoltzfus Patrick M. Woodward |
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Affiliation: | a Reed College, 3203 SE Woodstock Blvd. Portland, OR 97202, USA b Brockhouse Institute for Materials Research and Department of Chemistry, McMaster University, Hamilton, Ont., Canada, L8S 4M1 c Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK d Department of Chemistry, Ohio State University, Columbus, OH 43210, USA |
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Abstract: | Reduced titanates in the ATi2O4 (A=Li, Mg) spinel family exhibit a variety of interesting electronic and magnetic properties, most notably superconductivity in the mixed-valence spinel, Li1+xTi2−xO4. The sodium and calcium analogs, NaTi2O4 and CaTi2O4, each differ in structure, the main features of which are double rutile-type chains composed of edge-sharing TiO6 octahedra. We report for the first time, the properties and band structures of these two materials. XANES spectroscopy at the Ti K-edge was used to probe the titanium valence. The absorption edge position and the pre-edge spectral features observed in the XANES data confirm the assignment of Ti3+ in CaTi2O4 and mixed-valence Ti3+/Ti4+ in NaTi2O4. Temperature-dependent resistivity and magnetic susceptibility studies are consistent with the classification of both NaTi2O4 and CaTi2O4 as small band-gap semiconductors, although changes in the high-temperature magnetic susceptibility of CaTi2O4 suggest a possible insulator-metal transition near 700 K. Band structure calculations agree with the observed electronic properties of these materials and indicate that while Ti-Ti bonding is of minimal importance in NaTi2O4, the titanium atoms in CaTi2O4 are weakly dimerized at room temperature. |
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Keywords: | Mixed-valence Titanate Xanes spectroscopy Electronic structure Band structure Magnetic susceptibility Electrical resistivity |
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