Jacob, K. T. ; Sriram, M. V. (1994) Phase relations and gibbs energies in the system Mn-Rh-O Metallurgical and Materials Transactions A, 25 (7). pp. 1347-1357. ISSN 1073-5623
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Related URL: http://dx.doi.org/10.1007/BF02665468
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 + Mn3-2xRh2xO4, 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.
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Deposited On: | 11 Nov 2010 07:09 |
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