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Phase Relations in the System Cu─Ho─O and Stability of Cu 2 Ho 2 O 5
Author(s) -
Mathews Tom,
Jacob K. Thomas
Publication year - 1994
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1994.tb06966.x
Subject(s) - endothermic process , analytical chemistry (journal) , phase diagram , partial pressure , oxygen , electron microprobe , crystallography , copper , atmospheric temperature range , solid solution , chemistry , phase (matter) , mineralogy , thermodynamics , physics , organic chemistry , adsorption , chromatography
The phase relations in the system Cu─Ho─O have been determined at 1300 K using X‐ray diffraction, optical microscopy, and electron microprobe analysis of samples equilibrated in evacuated quartz ampules and in pure oxygen. Only one ternary compound, Cu 2 Ho 2 O 5 , was found to be stable. The Gibbs free energy of formation of this compound has been measurer using the solid‐state cellPt, Cu 2 O + Cu 2 Ho 2 O 5 + Ho 2 O 3 /(Y 2 O 3 )ZrO 2 /CuO + Cu 2 O, Ptin the temperature range of 973 to 1350 K. For the formation of Cu 2 Ho 2 O 5 from its binary component oxides,2CuO( s ) + Ho 2 O 3 ( s ) → Cu 2 Ho 2 O 5 ( s )Δ G °= 11190 − 13.8 T (±120)J·mol −1 Since the formation is endothermic, Cu 2 Ho 2 O 5 becomes thermodynamically unstable with respect to CuO and Ho 2 O 3 below 810 K. When the oxygen partial pressure over Cu 2 Ho 2 O 5 is lowered, it decomposes according to the reaction2Cu 2 Ho 2 O 5 ( s ) → 2Ho 2 O 3 ( s ) + 2Cu 2 O( s ) + O 2 (g)for which the equilibrium oxygen potential is given byΔμo 2 =−238510 + 160.2 T (±450) J·mol −1 The decomposition temperature at an oxygen partial pressure of 1.52 × 10 4 Pa was measured using a combined DTA–TGA apparatus. Based on these results, an oxygen potential diagram for the system Cu─Ho─O at 1300 K is presented.