CA0.95NB3O6 - CRYSTAL AND ELECTRONIC-STRUCTURE

The title compound has been synthesized and its structure refined by using powder diffraction data. The structure is orthorhombic, space group Immm, with cell constants a = 7.1156 (9), b = 10.2863 (18), c = 6.5756 (9) Å, and Z = 4. Two types of Nb atoms are present in the structure: Nb1 atoms in Nb2O8 clusters with one of the shortest Nb-Nb distances known so far (2.560 Å), and Nb2 atoms forming layers of connected NbO6 octahedra. The electronic structure obtained from Extended Hückel tight-binding band calculations suggests the coexistence of Nb(1+) ions in the Nb2O8 clusters with quadruple Nb-Nb bonds, together with Nb(4.5+) ions in the layers of octahedra. Deviations from this simplified model are analyzed. The electrical behavior, measured in the temperature range 4.2 K < T < 300 K, is metallic at low temperatures and practically temperature-independent at room temperature. A semimetallic character is proposed based on the calculated band electronic structure, with the states around the Fermi level localized mainly on the layers of octahedra parallel to the ac plane. © 1993 Academic Press, Inc.