CuZr2(PO4)(3) crystallises with the Nasicon-type structure and is a copper(l) 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) + 1/2CuO (s) <-> CuZr2(PO4)(3) (s) + 1/4O(2)(g) was studied by thermogravimetry in an atmosphere P-O2 = 0.22 atm and was found to occur at 475 +/- 10 degrees C. Thus, CuZr2(PO4)(3) is a thermodynamically stable phase in air above similar to 475 degrees 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 degrees 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 clopant 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. (c) 2006 Springer Science + Business Media, Inc.