Phase Equilibria in the System CuO‐NiO‐P4O10 and Synthesis,Crystal Structure,and Characterization of the New Copper Nickel Oxide Phosphate Cu3NiO(PO4)2

The system CuO‐NiO‐P₄O₁₀ was investigated using a solid state reaction between CuO, NiO, and (NH₄)₂HPO₄ 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 Cu₃NiO(PO₄)₂ were achieved by cooling the melted compound in a sealed, evacuated quartz ampoule. Cu₃NiO(PO₄)₂ crystallizes in the monoclinic space group P2₁/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 Cu1O₅], distorted planar squares Cu2O₄], octahedra Cu3O₆], and NiO₆] and PO₄] 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.