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 Na 4 Ni 5 (PO 4 ) 2 (P 2 O 7 ) 2 have been isolated and their structure has been determined by X-ray diffraction techniques. This compound crystallizes in the monoclinic space group P 2 1 / 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 (Ni 5 P 2 O 22 ) ∞ 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 (Ni 5 P 2 O 22 ) ∞ blocks, which host the sodium cations. The framework is built up from PO 4 tetrahedra, P 2 O 7 groups, NiO 6 octahedra, and Ni 2 O 9 units formed by face-sharing between two NiO 6, which is unusual in these kinds of compounds. Magnetic measurements reveal the presence of antiferromagnetic interactions in the Ni 2+ 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 1−10 5 Hz. The determined activation energy was 0.99 eV and the conductivity at 500 °C was 2.8 × 10 - 6 S cm - 1 .