Synthesis and properties of copper-hafnium triphosphate CuIHf2(PO4)3

Monovalent copper-hafnium triphosphate CuIHf2(PO4)3 was obtained in a single phase using sol-gel type methods and solid state reactions. It was characterized by X-ray diffraction, 31P MAS-NMR and diffuse reflectance spectroscopies. Its crystallographic parameters were found to be similar to those of Nasicon type compounds CuIM2(PO4)3 (M = Ti, Sn, Zr). Moreover, results of the thermogravimetric analysis and EDX-STEM microscopy were consistent with the structure features and the existence of the reversible redox system Cu2+Cu+ in the network of the phosphate. Under the electronic beam of the microscope, the Cu+ ions diffuse throughout the conduction tunnels to give on the surface of the material crystallites and whiskers of metallic copper. This migration process does not affect notably the structure of the phosphate.