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# Solid State Cells with Buffer Electrodes for Measurement of Chemical Potentials and Gibbs Energies of Formation: System Ca-Rh-O

Jacob, KT and Waseda, Y (2000) Solid State Cells with Buffer Electrodes for Measurement of Chemical Potentials and Gibbs Energies of Formation: System Ca-Rh-O. In: Journal of Solid State Chemistry, 150 (1). pp. 213-220.

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## Abstract

The isothermal section of the phase diagram for the system Ca-Rh-O at 1273 K has been determined by equilibrating 12 compositions in the ternary and identifying phases present in quenched samples by optical microscopy, powder X-ray diffraction(XRD), and energy-dispersive analysis of X rays (EDX). One ternary compound, $CaRh_2O_4$, was found to be stable. It coexists with CaO and metallic Rh. All the alloy compositions are in equilibrium with CaO. The standard Gibbs energy of formation of $CaRh_2O_4$ and the oxygen chemical potential corresponding to the three-phase field Rh+CaO+$CaRh_2O_4$ were determined using novel designs of solid state cells incorporating yttria-stabilized zirconia as the electrolyte: Pt-Rh, CaO+$CaRh_2O_4$ Rh/$(Y_2O_3)$$ZrO_2/Rh+Rh_2O_3, Pt-Rh, Pt+Rh, CaO+CaRh_2O_4+Rh/(Y_2O_3)$$ZrO_2$/$O_2$(0.1 MPa),Pt-Rh. Buffer electrodes were introduced between reference and working electrodes to dissipate the electrochemical flux of oxygen through the solid electrolyte caused by the difference in chemical potential of oxygen at the electrodes. Polarization of the electrodes was prevented by the use of buffer electrodes. For the reaction $2Rh+CaO+\frac{2}{3}O_2 \rightarrow CaRh_2O_4$, $\Delta G^o=\frac{2}{3} \Delta \mu_{02}$ =-493,340+288.67T $(\pm 285)$ J/mol. The Gibbs energy of formation of the interoxide compound from the component binary oxides is $\beta-Rh_2O_3+CaO$ $\rightarrow CaRh_2O_4$, $\Delta G^o=-101,670+10.6T (\pm 290) J/mol$. Using the Neumann-Kopp rule to estimate the heat capacity of $CaRh_2O_4$, the standard enthalpy of formation and standard entropy of the compound at 298.15 K are evaluated as -1142.3$(\pm 2)$ kJ/mol and 103.2$(\pm 2)$ J/mol'K, respectively.

Item Type: Journal Article Copyright of this article belongs to Academic Press phase diagram, system Ca-Rh-O;calcium rhodite,CaRh2O4;Gibbs energy, enthalpy, entropy of formation;decomposition temperature. Division of Chemical Sciences > Materials Research CentreDivision of Mechanical Sciences > Materials Engineering (formerly Metallurgy) 02 Aug 2006 19 Sep 2010 04:30 http://eprints.iisc.ernet.in/id/eprint/7979

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