Synthesis and properties of a new copper(II)-hafnium phosphate Cu 0.5Hf 2(PO 4) 3

• Published in: Solid state ionics Vol. 110; no. 3; pp. 311 - 318
• Main Authors: Ziyad, Mahfoud, Ahmamouch, Rachid, Rouimi, Mohamed, Gharbage, Slimane, Védrine, Jacques C.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 1998
• Subjects: Copper–hafnium phosphate; Electronic paramagnetic resonance; Nuclear magnetic resonance; Temperature-programmed reduction and oxidation; X-ray diffraction; Temperature-programmed reduction and oxidation; Electronic paramagnetic resonance; X-ray diffraction; Nuclear magnetic resonance; Copper–hafnium phosphate
• ISSN: 0167-2738; 1872-7689
• DOI: 10.1016/S0167-2738(98)00139-8

Phosphates of general formula M 0.5Hf 2(PO 4) 3 with M=Cd 2+, Ca 2+, Sr 2+ and Cu 2+ were prepared by coprecipitation and characterized by several physical techniques. The compounds containing Cd 2+, Ca 2+, Sr 2+ belong to the Nasicon-type structure, whereas Cu 0.5Hf 2(PO 4) 3 exhibited substantially different DRX patterns. Combined temperature programmed reduction (TPR) and temperature-programmed oxidation (TPO) showed that the copper in Cu 0.5Hf 2(PO 4) 3 was distributed between two energetically different sites in proportions respectively equal to 40 and 60%. Electron Paramagnetic Resonance (EPR) investigations confirmed the TPR/TPO results and revealed that the two sites hosting the Cu 2+ ions are of orthorhombic symmetry. Moreover, the Cu 2+ ions might be reduced by hydrogen to Cu +. These results were also supported by the UV–visible studies that showed the disappearance, under reducing conditions, of the band corresponding to crystal field transitions of Cu 2+ ions and the emergence of a new peak attributed to the transitions between (3d) 10 and (3d) 9(4s) 1 Cu + levels. At the same time, IR spectroscopy confirmed that protons entered the open lattice framework of the material and gave rise to a new protonated phase containing monovalent copper Cu 0.5 IH 0.5Hf 2(PO 4) 3. This redox process was proven to be reversible without any subsequent change in the network of the phosphate.