Contributions on the thermal behaviour of oxoniobates of the transition metals. Pt. 2

Crystals of MnNb2O6 were obtained by chemical transport reactions in a temperature gradient (1020degC→960degC) using Cl2 (added as PtCl2) or NH4Cl as transport agent. As a result of thermodynamic calculations the evaporation and deposition of MnNb2O6 in the present of Cl2 can be expressed by the endothermic equilibrium (1). MnNb2O6,s + 4Cl2,g = MnCl2,g + 2NbOCl3,g + 2O2,g The endothermic reaction (2) is responsible for the migration of MnNb2O6 if NH4Cl is used as transport agent. MnNb2O6,s + 8HClg = MnCl2,g + 2NbOCl3,g + 4H2Og Assuming ΔBH298deg(MnNb2O6,s)=-567.6 kcal/mol a satisfying agreement between thermodynamic calculations and experimental results can be reached. (orig.)