Phase Equilibria in the System CuO‐NiO‐P 4 O 10 and Synthesis, Crystal Structure, and Characterization of the New Copper Nickel Oxide Phosphate Cu 3 NiO(PO 4 ) 2

The system CuO‐NiO‐P 4 O 10 was investigated using a solid state reaction between CuO, NiO, and (NH 4 ) 2 HPO 4 in quartz crucibles at 900 °C. The powder samples were characterized by X‐ray diffraction, TG/DTA, electrochemical measurements, IR, and UV/Vis spectroscopy. Single crystals of a new quaternary phase Cu 3 NiO(PO 4 ) 2 were achieved by cooling the melted compound in a sealed, evacuated quartz ampoule. Cu 3 NiO(PO 4 ) 2 crystallizes in the monoclinic space group P 2 1 / n (no 14) with a = 8.2288(2) Å, b = 9.8773(2) Å, c = 8.2777(3) Å, β = 107.82(2)°, Z = 4. The three‐dimensional framework consists of distorted tetragonal pyramides [Cu1O 5 ], distorted planar squares [Cu2O 4 ], octahedra [Cu3O 6 ], and [NiO 6 ] and [PO 4 ] tetrahedra. The TG‐DTA of the new phase showed an incongruent melting at 1055 °C. The open circuit voltage of this material was measured to determine the electrochemical properties. The measurement revealed an initial capacity of 236 Ah · g –1 and a voltage plateau at 2.05 V. Furthermore, it was possible to identify the phase equilibria and to obtain the phase diagram at 900 °C.