Phase relations and Gibbs energies of spinel phases and solid solutions in the system Mg–Rh–O

Pure stoichiometric MgRh2O4 could not be prepared by solid state reaction from an equimolar mixture of MgO and Rh2O3 in air. The spinel phase formed always contained excess of Mg and traces of Rh or Rh2O3. The spinel phase can be considered as a solid solution of Mg2RhO4 in MgRh2O4. The compositions of the spinel solid solution in equilibrium with different phases in the ternary system Mg–Rh–O were determined by electron probe microanalysis. The oxygen potential established by the equilibrium between Rh + MgO + Mg1+xRh2−xO4 was measured as a function of temperature using a solid-state cell incorporating yttria-stabilized zirconia as an electrolyte and pure oxygen at 0.1 MPa as the reference electrode. To avoid polarization of the working electrode during the measurements, an improved design of the cell with a buffer electrode was used. The standard Gibbs energies of formation of MgRh2O4 and Mg2RhO4 were deduced from the measured electromotive force (e.m.f.) by invoking a model for the spinel solid solution. The parameters of the model were optimized using the measured composition of the spinel solid solution in different phase fields and imposed oxygen partial pressures. The results can be summarized by the equations: MgO + β–Rh2O3 → MgRh2O4; ΔG∘(± 1010)/J mol−1 = − 32239 + 7.534T; 2MgO + RhO2 → Mg2RhO4; ΔG∘(± 1270)/J mol−1 = 36427 − 4.163T; ΔGM/J mol−1 = 2RT(x In x + (1 − x)In(1 − x)) + 4650x(1 − x),where ΔG° is the standard Gibbs free energy change for the reaction and ΔGM is the free energy of mixing of the spinel solid solution Mg1+xRh2−xO4.

► Refinement of phase diagram for the system Mg-Rh-O and thermodynamic data for spinel compounds MgRh2O4 and Mg2RhO4 is presented.
► A solid-state electrochemical cell is used for thermodynamic measurement.
► An advanced design of the solid-state electrochemical cell incorporating buffer electrodes is deployed to minimize polarization of working electrode.
► Regular solution model for the spinel solid solution MgRh2O4 – Mg2RhO4 based on ideal mixing of cations on the octahedral site is proposed.
► Factors responsible for stabilization of tetravalent rhodium in spinel compounds are identified.