NICKEL SUPERMIXED VALENCE IN STOICHIOMETRIC BANI0.83O2.5

Single crystals of BaNi0.83O2.5 were grown from mixtures of BaCO3 and NiO using KOH as a flux. The composition and the crystal structure have been established from X-ray single crystal diffraction data. BaNi0.83O2.5 is stoichiometric and crystallizes in a trigonal structure, space group R32, with hexagonal unit-cell parameters (Å) a = 9.8890(4) c = 12.867(4), volume 1089.7(3) Å3, and Z = 18. BaNi0.83O2.5 contains two kinds of Ba atoms both coordinated with 10 oxygens, and three different Ni atoms octahedrally coordinated to oxygen at distances which approximately correspond to Ni2+ and to low spin Ni3+ and Ni4+. NiO6 octahedra share opposite faces giving rise to chains parallel to the c axis, in which some Ni-Ni distances are shorter than in Ni metal. This structure, which is compared with that of BANiO3, can be considered like a close packing of Ba and oxygen atoms in which almost one quarter of the octahedral sites are occupied by nickels. BaNi0.83O2.5 is an electrical insulator. The results of magnetic measurements from room temperature to 5 K are given. © 1994 Academic Press, Inc.