Gibbs free energy of formation of calcium rhodite

The Gibbs free energy of formation of CaRh₂O₄(s) has been determined using two techniques viz., quadrupole mass spectrometer coupled to a Knudsen cell and solid-state cell incorporating CaF₂(s) as the solid electrolyte. In the former method, equilibrium O₂(g) pressures were measured over the phase field Rh(s)+Rh₂O₃(s), in the temperature range 793.7-909.1 K and over the three phase mixture CaRh₂O₄(s)+Rh(s)+CaO(s) was measured from 862.1 to 1022.7 K.The Gibbs free energy of formation of Rh₂O₃(s) from elements in their standard state can be given by