Stability of Cu₂Ln₂O₅ compounds: Comparison, assessment and systematics

Abstract

Phase diagram studies show that at ambient pressure only one ternary oxide, Cu₂Ln₂O₅, is stable in the ternary systems Cu-Ln-O (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu) at high temperatures. The crystal structure of Cu₂Ln₂O₅ can be described as a zig-zag arrangement of one-dimensional Cu₂O₅ chains parallel to the a-axis with Ln atoms occupying distorted octahedral sites between these chains. Four sets of emf measurements on Gibbs energy of formation of Cu₂Ln₂O₅ (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu; Y) from component binary oxides and one set of high-temperature solution calorimetric data on enthalpy of formation have been reported in the literature. Except for Cu₂Y₂O₅, the measured values for the Gibbs energies of formation of all other Cu₂Ln₂O₅ compounds fall in a narrow band (±1 kJ mol^-1) and indicate a regular increase in stability with decreasing ionic radius of the lanthanide ion. The values for the second law enthalpy of formation, derived from the temperature dependence of emf obtained in different studies, show larger differences, as high as 25 kJ mol^-1 for Cu₂Tm₂O₅. Though associated with an uncertainty of ±4 kJ mol^-1, the calorimetric measurements help to identify the best set of emf data. The trends in thermodynamic data correlate well with the global instability index (GII) based on the overall deviation from the valence sum rule. Low values for the index calculated from crystallographic information indicate higher stability. Higher values are indicative of the larger stress in the structure.