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

Jacob, KT and Prusty, Debadutta and Kale, GM (2012) Phase relations and Gibbs energies of spinel phases and solid solutions in the system Mg-Rh-O. In: Journal of Alloys and Compounds, 513 . pp. 365-372.

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Official URL: http://dx.doi.org/10.1016/j.jallcom.2011.10.050


Pure stoichiometric MgRh(2)O(4) could not be prepared by solid state reaction from an equimolar mixture of MgO and Rh(2)O(3) in air. The spinel phase formed always contained excess of Mg and traces of Rh or Rh(2)O(3). The spinel phase can be considered as a solid solution of Mg(2)RhO(4) in MgRh(2)O(4). 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 + Mg(1+x)Rh(2-x)O(4) 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 MgRh(2)O(4) and Mg(2)RhO(4) 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 + beta -Rh(2)O(3) -> MgRh(2)O(4); Delta G degrees (+ 1010)/J mol(-1) = -32239 + 7.534T; 2MgO + RhO(2) -> Mg(2)RhO(4); Delta G degrees(+/- 1270)/J mol(-1) = 36427 -4.163T; Delta G(M)/J mol(-1) = 2RT(xInx + (1-x)In(1-x)) + 4650x(1-x), where Delta G degrees is the standard Gibbs free energy change for the reaction and G(M) is the free energy of mixing of the spinel solid solution Mg(1+x)Rh(2-x)O(4). (C) 2011 Elsevier B. V. All rights reserved.

Item Type: Journal Article
Publication: Journal of Alloys and Compounds
Publisher: Elsevier Science
Additional Information: Copyright of this article belongs to Elsevier Science.
Keywords: Thermodynamic properties; Electromotive force (e.m.f.); Free energy;Enthalpy;Entropy;Stability;Spinels;Solid solution;Phase diagram;Magnesium rhodite (MgRh(2)O(4)); Magnesium rhodate (Mg(2)RhO(4))
Department/Centre: Division of Mechanical Sciences > Materials Engineering (formerly Metallurgy)
Date Deposited: 27 Jan 2012 12:19
Last Modified: 27 Jan 2012 12:19
URI: http://eprints.iisc.ac.in/id/eprint/43201

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