Crystal Structure, Magnetic Properties, and Ionic Conductivity of a New Mixed-Anion Phosphate Na4Ni5(PO4)2(P2O7)2

Single crystals of the new sodium phosphate Na4Ni5(PO4)2(P2O7)2 have been isolated and their structure has been determined by X-ray diffraction techniques. This compound crystallizes in the monoclinic space group P21/c with a = 12.544(2) Å, b = 6.596(1) Å, c = 10.572(2) Å, β = 103.731(3)°, and Z = 2. Its three-dimensional structure can be described as formed by (Ni5P2O22)∞ infinite blocks parallel to the bc plane being the interblock linkages via P−O−P bridges of the diphosphate groups. There are tunnels along the [100] direction, crossing the (Ni5P2O22)∞ blocks, which host the sodium cations. The framework is built up from PO4 tetrahedra, P2O7 groups, NiO6 octahedra, and Ni2O9 units formed by face-sharing between two NiO6, which is unusual in these kinds of compounds. Magnetic measurements reveal the presence of antiferromagnetic interactions in the Ni2+ sublattice, at about 8 K. The ionic conductivity, due to the motion of sodium ions, was measured at different temperatures (300−700 °C) in the frequency range 1−105 Hz. The determined activation energy was 0.99 eV and the conductivity at 500 °C was 2.8 × 10-6 S cm-1.