Solid state reactivity and mechanisms in oxide systems

Starting from oxalate mixed crystals Ni_xMn_3?x(C₂O₄)₃·6H₂O thermal decomposition at reduced oxygen partial pressure (po₂= 2%) leads to the formation of NiMn₂O₄ (x = 1) at metastable conditions. Ni_1.5Mn_1.5O₄ (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 1/2α-Mn₂O₃ or NiMnO₃ only or MgTiO₃ and MgO, respectively. The reaction rates observed are lower for NiMn₂O₄ and Ni_1.5Mn_1.5O₄ than for Mg₂TiO₄ decomposition. The reverse reaction of NiMn₂O₄ formation above 730°C shows an endothermic enthalpy of +61 kJ·mol^?1. For Mg₂TiO₄ formation above 1050°C an endothermic enthalpy of +19.3 kJ·mol^?1 is found. The results are discussed in terms of structural features of the oxides.