Phase relations and gibbs energies in the system Mn-Rh-O

Abstract

Phase relations in the system Mn-Rh-O are established at 1273 K by equilibrating different compositions either in evacuated quartz ampules or in pure oxygen at a pressure of 1.01 × 10⁵ Pa. The quenched samples are examined by optical microscopy, X-ray diffraction, and energy-dispersive X-ray analysis (EDAX). The alloys and intermetaUics in the binary Mn-Rh system are found to be in equilibrium with MnO. There is only one ternary compound, MnRh₂0₄, with normal spinel structure in the system. The compound Mn₃0₄ has a tetragonal structure at 1273 K. A solid solution is formed between MnRh₂0₄ and Mn₃0₄. The solid solution has the cubic structure over a large range of composition and coexists with metallic rhodium. The partial pressure of oxygen corresponding to this two-phase equilibrium is measured as a function of the composition of the spinel solid solution and temperature. A new solid-state cell, with three separate electrode compartments, is designed to measure accurately the chemical potential of oxygen in the two-phase mixture, Rh + Mn_3-2xRh_2xO₄, which has 1 degree of freedom at constant temperature. From the electromotive force (emf), thermodynamic mixing properties of the Mn₃0₄-MnRh₂0₄ solid solution and Gibbs energy of formation of MnRh₂0₄ are deduced. The activities exhibit negative deviations from Raoult's law for most of the composition range, except near Mn304, where a two-phase region exists. In the cubic phase, the entropy of mixing of the two Rh³⁺ and Mn³⁺ ions on the octahedral site of the spinel is ideal, and the enthalpy of mixing is positive and symmetric with respect to composition. For the formation of the spinel (sp) from component oxides with rock salt (rs) and orthorhombic (orth) structures according to the reaction, MnO (rs) + Rh₂0₃ (orth) → MnRh₂0₄ (sp), ΔG° = -49,680 + 1.56T (±500) J mol^-1. The oxygen potentials corresponding to MnO + Mn₃0₄ and Rh + Rh₂0₃ equilibria are also obtained from potentiometric measurements on galvanic cells incorporating yttria-stabilized zirconia as the solid electrolyte. From these results, an oxygen potential diagram for the ternary system is developed.