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

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