SOLID-STATE REACTIVITY AND MECHANISMS IN OXIDE SYSTEMS .9. FORMATION AND DECOMPOSITION OF NIMN2O4, NI1.5MN1.5O4 AND MG2TIO4 IN THE METASTABLE STATE

Starting from oxalate mixed crystals NixMn3-x(C2O4)3.6H2O thermal decomposition at reduced oxygen partial pressure (pO2=2%) leads to the formation of NiMn2O4 (x = 1) at metastable conditions. Ni1.5Mn1.5O4 (x = 1.5) existing in the metastable state only has been also prepared. The spinel compounds both are of the highly inversed type. Following a sol-gel preparation route Mg2TiO4 has been also found to be formed in the metastable state. Annealing results in decomposition of the compounds providing NiMnO3 and 1/2alpha-Mn2O3 or NiMnO3 only or MgTiO3 and MgO, respectively. The reaction rates observed are lower for NiMn2O4 and Ni1.5Mn1.5O4 than for Mg2TiO4 decomposition. The reverse reaction of NiMn2O4 formation above 730-degrees-C shows an endothermic enthalpy of +61 kJ.mol-1. For Mg2TiO4 formation above 1050-degrees-C an endothermic enthalpy of +19.3 kJ.mol-1 is found. The results are discussed in terms of structural features of the oxides.