PREPARATION AND THERMOCHEMICAL PROPERTIES OF BANIO2+X

Single crystals of BaNiO3 have been synthesized in molten KOH. The enthalpies of formation from the component oxides at 25-degrees-C Of BaNiO2, BaNiO2.63, and BaNiO3 have been measured using high temperature reaction calorimetry. As the value of x increases in BaNiO2+x, the enthalpy of formation becomes more exothermic. From the enthalpy of formation data, an enthalpy of oxidation of -196 +/- 20 kJ/(mole O2) was obtained. This value is very similar to the values of -202 +/- 18 and -220 +/- 50 kJ/(mole O2) obtained for YBa2Cu3O7-y and La2-xBaxCuO4-y, respectively Z. Zhou, and A. Navrotsky, J. Mater. Res. 7(11), 2920 (1992); J. DiCarlo, J. Bularzik, and A. Navrotsky, J. Solid State Chem. 96, 381 (1992). Energetic and structural data are discussed in terms of a model in which the holes have significant oxygen character [A. Mehta, J. DiCarlo, and A. Navrotsky, J. Solid State Chem. 101, 173 (1992).] (C) 1994 Academic Press, Inc.