Stability of Cu2Ln2O5 compounds: Comparison, assessment and systematics

Jayadevan, K. P. ; Jacob, K. T. (2000) Stability of Cu2Ln2O5 compounds: Comparison, assessment and systematics High-temperature materials and processes, 19 (6). pp. 389-397. ISSN 0334-6455

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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.

Item Type:Article
Source:Copyright of this article belongs to Walter de Gruyter GmbH & Co. KG.
ID Code:95013
Deposited On:09 Nov 2012 06:35
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