Phase relations and gibbs energies in the system Mn-Rh-O
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I.
INTRODUCTION
T H E R E is currently much interest in the compound MnRh204 and the spinel solid solution Mn3_2~Rh2~O4 for catalytic applications. A partial phase diagram for the system Mn-Rh-O at 1373 K has been proposed by Schmahl et al. m They indicate a complete range of solid solution between Mn304 with tetragonal structure and MnRh204 with cubic structure, which cannot be valid. The purpose of this study is to establish a complete isothermal section of the ternary phase diagram at 1273 K and to measure thermodynamic properties of ternary oxide phases as a function of composition and temperature. The phase diagram is established by identification of phases present in equilibrated samples. The different compositions are prepared by mixing metals, alloys, and oxides in different proportions. The samples containing only oxides in their highest oxidation state are equilibrated in pure oxygen. Samples containing alloys and lower-valent oxides are equilibrated in evacuated quartz ampules to avoid oxidation. The sample is contained in
K.T. JACOB, Chairman and Professor, Materials Research Center, and Professor, Department of Metallurgy, is with the Indian Institute of Science, Bangalore-560 012, India. M.V. SRIRAM, Student, is with the Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur 208 016, India. Manuscript submitted October 4, 1993. METALLURGICALAND MATERIALS TRANSACTIONSA
a zirconia crucible placed inside the quartz ampule. Isothermal equilibration studies are supplemented by differential thermal analysis (DTA) runs on Mn3_2~Rhz~O4 solid solutions, for small values of x. After establishing phase relations in the system MnRh-O, a solid-state galvanic cell is designed to measure the thermodynamic properties of ternary oxide phases. Because of the nature of phase relations in the Mn-Rh-O system, Gibbs energy of formation of MnRh204 cannot be directly obtained from electromotive force (emf) measurement involving three-phase equilibria. The Gibbs energy of formation is derived from measurements on spinel solid solutions. The solid solutions exist with metallic rhodium. The two-phase region has associated with it 1 degree of freedom at constant temperature. The composition of the spinel changes during emf measurement of the oxygen chemical potential because of electrochemical permeability of the solid electrolyte. A special galvanic cell, incorporating three compartments, is designed to overcome this problem. In addition to the usual measuring and reference electrodes, a third buffer electrode is introduced which acts as a sink for the flux of oxygen. A modified form of the Gibbs-Duhen relation is developed for deriving the activity coefficients of MnRh204 and Mn304 from the variation of the equilibrium partial pressure of oxygen with composition of the spinel solid solution. The equilibrium constant for the reaction VOLUME 25A, JULY 1994--1347
1/202
represented by the tie lines is then derived from the oxygen partial pressure and activities in the solid solution. The standard Gib
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