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

Jacob, K. T. ; Prusty, Debadutta ; Kale, G. M. (2012) Phase relations and Gibbs energies of spinel phases and solid solutions in the system Mg–Rh–O Journal of Alloys and Compounds, 513 . pp. 365-372. ISSN 0925-8388

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Official URL: http://www.sciencedirect.com/science/article/pii/S...

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

Abstract

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.

Item Type:Article
Source: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 (MgRh2O4); Magnesium Rhodate (Mg2RhO4)
ID Code:95079
Deposited On:30 Oct 2012 09:26
Last Modified:30 Oct 2012 09:26

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