Can the structure of the Ti or V Magneli binary oxides host superconductivity?

A strategy for tailoring new superconducting systems is based on band struc ture calculations and the use of the empirical criteria suggested by L.F. M attheiss. These criteria allow one to evaluate the capability of new materi als to host superconductivity. Promising host structures might be found amo ng those materials undergoing metal-insulator transitions as a function of temperature. Materials with metallic ground states can then be generated by substitution. In this article, we suggest that simple binary compounds, su ch as titanium or vanadium oxides belonging to the homologous series MnO2n- 1 may be good candidates. They exhibit structures which can be derived from rutile by removing one oxygen atom every n M cation via a shearing mechani sm, which besides creating n-octahedron-thick rutile blocks along the pseud o tetragonal c-axis, generates a non-stoichiometry with respect to the MO2 formula. Because of the mixed valence state, these materials generally exhi bit metallic conductivity and undergo metal-to-insulator transitions as a f unction of temperature. For example, Ti4O7 undergoes a transition at 150 K, at which temperature a drop of conductivity by three orders of magnitude a nd one of magnetic susceptibility by a factor of three are observed. On fur ther cooling, a second drop of conductivity by two orders of magnitude is o bserved at 130 K when the compound undergoes a semiconductor-to-semiconduct or transition. The V counterpart seems to be a somewhat better candidate th an Ti4O7. The behavior of V4O7 as function of temperature has some similari ty to that of undoped La2CuO4 . The vanadium Magneli phase with 50% V3+ and 50% V3+, has a metallic conductivity above 250 K, becomes a semiconductor below this temperature and orders antiferromagnetically at 40 K. (C) 2000 E lsevier Science B.V. All rights reserved.