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

Abstract Reduced titanates in the A Ti 2 O 4 ( A =Li, Mg) spinel family exhibit a variety of interesting electronic and magnetic properties, most notably superconductivity in the mixed-valence spinel, Li 1+ x Ti 2− x O 4 . The sodium and calcium analogs, NaTi 2 O 4 and CaTi 2 O 4 , each differ in structure, the main features of which are double rutile-type chains composed of edge-sharing TiO 6 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 Ti 3+ in CaTi 2 O 4 and mixed-valence Ti 3+ /Ti 4+ in NaTi 2 O 4 . Temperature-dependent resistivity and magnetic susceptibility studies are consistent with the classification of both NaTi 2 O 4 and CaTi 2 O 4 as small band-gap semiconductors, although changes in the high-temperature magnetic susceptibility of CaTi 2 O 4 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 NaTi 2 O 4 , the titanium atoms in CaTi 2 O 4 are weakly dimerized at room temperature.