A study of copper stoichiometry and phase relationships in the copper-zirconium phosphate system: CuZr2(PO4)3 – Cu0.5Zr2(PO4)3
2006
Christiansen, R. H.-W. | Warner, T. E.
CuZr₂(PO₄)₃ crystallises with the Nasicon-type structure and is a copper(I) ion conductor. The possibility of a solid solution between CuZr₂(PO₄)₃ and Cu₀.₅Zr₂(PO₄)₃ has been a controversial issue for many years. As part of a continued study, CuZr₂(PO₄)₃ and Cu₀.₅Zr₂(PO₄)₃ were prepared by solid state methods and used to investigate the copper stoichiometry and phase relationships between these two materials as a function of copper content, temperature and oxygen fugacity. The following reversible reaction: Cu₀.₅Zr₂(PO₄)₃ (s) + [Formula: see text]CuO (s) ↔ CuZr₂(PO₄)₃ (s) + [Formula: see text]O₂(g) was studied by thermogravimetry in an atmosphere of PO₂ = 0.22 atm and was found to occur at 475 ± 10°C. Thus, CuZr₂(PO₄)₃ is a thermodynamically stable phase in air above ∼475°C, which places a lower temperature limit on its use as an electrolyte in air. The results of X-ray powder diffractometry on materials with various copper contents that had been annealed in argon at 750°C indicate that there is no evidence for a significant solid solution between CuZr₂(PO₄)₃ and Cu₀.₅Zr₂(PO₄)₃ nor, a reductive decomposition of Cu₀.₅Zr₂(PO₄)₃. The coexistence of CuZr₂(PO₄)₃ and Cu₀.₅Zr₂(PO₄)₃ as discrete phases is also supported by evidence from electron spin resonance spectroscopy on these materials, which indicate the presence of copper(II) ions in CuZr₂(PO₄)₃ at a dopant and dispersed level of concentration. The results from energy dispersive X-ray analysis, as well as, the novel use of the fluorescent behaviour of CuZr₂(PO₄)₃ in ultra-violet light as an analytical tool, support the above conclusions.
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