Single crystal X-ray structure study of the Li2−xNaxNi[PO4]F system

Abstract

The new compounds Li_2−xNa_xNi[PO₄]F (x = 0.7, 1, and 2) have been synthesized by a solid state reaction route. Their crystal structures were determined from single-crystal X-ray diffraction data. Li_1.3Na_0.7Ni[PO₄]F crystallizes with the orthorhombic Li₂Ni[PO₄]F structure, space group Pnma, a = 10.7874(3), b = 6.2196(5), c = 11.1780(4) Å and Z = 8, LiNaNi[PO₄]F crystallizes with a monoclinic pseudomerohedrally twinned structure, space group P2₁/c, a = 6.772(4), b = 11.154(6), c = 5.021(3) Å, β = 90° and Z = 4, and Na₂Ni[PO₄]F crystallizes with a monoclinic twinned structure, space group P2₁/c, a = 13.4581(8), b = 5.1991(3), c = 13.6978(16) Å, β = 120.58(1)° and Z = 8. For x = 0.7 and 1, the structures contain NiFO₃ chains made up of edge-sharing NiO₄F₂ octahedra, whereas for x = 2 the chains are formed of dimer units (face-sharing octahedra) sharing corners. These chains are interlinked by PO₄ tetrahedra forming a 3D framework for x = 0.7 and different Ni[PO₄]F layers for x = 1 and 2. A sodium/lithium disorder over three atomic positions is observed in Li_1.3Na_0.7Ni[PO₄]F structure, whereas the alkali metal atoms are well ordered in between the layers in the LiNaNi[PO₄]F and Na₂Ni[PO₄]F structures, which makes both compounds of great interest as potential positive electrodes for sodium cells.