Gibbs' free energies of formation of Cu₂Ln₂O₅ (Ln=Tb, Dy, Er, Yb) compounds

Abstract

The standard Gibbs' free energies of formation of compounds of type Cu₂Ln₂O₅ (Ln = Tb,Dy,Er,Yb) were measured using the solid state cell in the temperature range of 970 to 1323 K For formation of Cu₂Ln₂O₅ compounds from their binary component oxides according to the reaction 2CUO(s)+ Ln0₃ (s) Cu₂Ln₂O₅ (s),the Gibbs' free energy changes can be represented by the following equations:AGO = 13 080 - 13.70 'I" (+80) J mol^-1 (Ln = Tb)AGq = 11 480 - 13.51 T (260) J mol^-1 (Ln = Dy)AGO = 10 750 - 13.99 T (260) J mol^-1 (Ln = Er)AGO = 9 920 - 13.90 T (260) J mol^-1 (Ln = Yb) Since formation of the compounds is endothermic, the compounds become thermodynamically unstable with respect to their component oxides below 955 K for Cu₂Ln₂O₅, 850 K for Cu₂Ln₂O₅, 768 K for Cu₂Ln₂O₅ and 714 K for Cu2Yb2OS When the oxygen partial pressure over Cu₂Ln0₅ is lowered, they decompose according to the scheme, 2CULnO, (s)-r₂L0₃ (s) ⁺² cu₂0⁺⁰²(g)The equilibrium chemical potentials of oxygen corresponding to the dissociation reactions are computed from the emf data and auxiliary information on Cu₂0 and CuO. The computed decomposition temperatures at an oxygen partial pressure of 5.0 x ld Pa are compared with those obtained directly from combined thermogravimetric (TGA) and differential thermal analyses (DTA).The free energy, enthalpy and entropy of formation of Cu2Ln205 compounds show systematic variation with the ionic radius of the trivalent lanthanide ion. The trends obtained in this study are compared with information available in the literature. The staZbility of Cu₂Ln₂O₅ compounds increases with the decrease in ionic radii of the Cu₂Ln₂O₅