Novel metal oxides prepared by ingenious synthetic routes

Several novel oxides have been prepared by the decomposition of carbonate precursors of calcite structure of the general formulas Mn-1 (-) xMxCO3 (M = Mg, Co, Cd), Ca1 - M-x(x)'CO3, and Ca1 - (x) M-- y(x)'M-y '' CO3 (M', M" = Mn, Fe, Co). Typical of the oxides synthesized are a variety of monoxides, Mn-1 (-) xMxO of rocksalt structure and perovskite-related oxides such as Ca2FeCoO5, Ca2Co2O5, and Ca3Fe2MnO8, many of them exhibiting novel anion-vacancy-ordered superstructures. An interesting series of oxides prepared by the topotactic reduction of Ca2Fe2 - x MnxO6 (-) (y) (synthesized by the decomposition of the corresponding carbonate precursors) is Ca2Fe2 - xMnxO5 where three different coordination polyhedra (octahedra, tetrahedra, and square pyramids) of the transition metals coexist. Topotactic reduction of LaCoO3 and LaNiO3 yields new oxides, La2Co2O5 and La2Ni2O5, possessing anion-vacancy-ordered superstructures. While La2Co2O5 adopts the brownmillerite structure, La2Ni2O5 exhibits a new type of vacancy ordering with octahedral and square-planar coordination of Ni2+. Slightly reduced LaNiO3 seem to exhibit crystallographic shear (CS) planes similar to WO3 - x. Topotactic dehydration of isostructural MO1 - xWxO3 center dot H2O provides a convenient route for the synthesis of Mo1 - xWx, O-3 (0 <= x <= 1) in ReO3-related structure. Dehydration of yellow MoO3 center dot H2O under mild conditions yields a new metastable MoO3 in ReO3-like structure. Slightly reduced Mo1 - xWxO3 exhibit disordered {102} CS planes. The Mo1 - xWxO3 prepared by dehydration of the monohydrates can be reduced at 750 degrees C in hydrogen to yield monophasic Mo-W alloys.