ELECTRIC AND MAGNETIC-PROPERTIES OF THE 1ST LAYERED CONDUCTING TITANIUM AND NIOBIUM OXIDES

Three-dimensional titanium and niobium oxides are superconducting at temperatures considerably lower than layered copper oxides. Therefore it seems worthwhile to study layered titanium and niobium oxides. Layered conducting phases, however, were not known until recently, when the study of the LaTiO(x) system led to the first layered conducting titanium oxides. Single crystals of isostructural niobium oxides, CaNbO(x), SrNbO(x) and Sr0.9La0.1NbO3.4, were prepared with the floating zone melting technique. In common with LaTiO(x) they have the following properties. For x congruent-to 3.4, its layered perovskite-related structure of a known type is derived from the layered highest-T(c) (> 1600 K) ferroelectrics LaTiO3.5, CaNbO3.5 and SrNbO3.5. In the range 3.42 < x < 3.50 well ordered intergrowth takes place between the ferroelectric x = 3.5 structure and the x = 3.4 structure. This intergrowth was found to be representative of the bulk and the stacking sequence is dependent on x. For x < 3.5, magnetic and resistivity measurements were performed between room temperature and 4.2 K. The temperature dependence of the magnetic susceptibility along the layers displays a minimum. Semiconducting behavior was found along and perpendicular to the layers. At low temperatures the resistivity exhibits thermally activated behavior with unexpectedly small activation energies in the meV range. This might indicate that a further lowering of the average titanium and niobium valencies could lead to metallic behavior.