Structure property relationships in the ATi2O4 (A=Na, Ca) family of reduced titanates

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.