Activity–Composition Relations in NiAl 2 O 4 ─MnAl 2 O 4 Solid Solutions and Stabilities of NiAl 2 O 4 and MnAl 2 O 4 at 1300° and 1400°C

Activities of NiO were measured in the oxide and spinel solutions of the system MnO–NiO–Al 2 O 3 at 1300° and 1400° C with the aim of deriving information on the thermodynamic properties of the spinel phases. Synthetic samples in selected phase assemblages of the system were equilibrated with metallic nickel and a gas phase of known oxygen partial pressures at a total pressure of 1 atm. The data on NiO activities and directions of conjugation lines between coexisting oxide and spinel phases were used to establish the activity–composition relations in spinel solid solutions at 1300° and 1400°C. The MnAl 2 O 4 –NiAl 2 O 4 solid solutions exhibit considerable negative deviations from ideality at these temperatures. The free energy of formation of MnAl 2 O 4 from its oxide components (MnO + Al 2 O 3 ) at 1300° and 1400°C is calculated to be −24.97 and −26.56 kJ. mol −1 , respectively. The activities determined in the stoichiometric spinel solid solutions are more negative as compared with those predicted from cation distribution models.