Na4.3Ni1.3Al1.7(PO4)4 structure transformation from stuffed α-CrPO4 type driven by the incorporation of Na: magnetism and ionic migration

Abstract

A new phase Na_4.3Ni_1.3Al_1.7(PO₄)₄ was prepared by hydrothermal synthesis. Its crystal structure was determined using single-crystal X-ray diffraction data and refined against F² to R = 0.055. The compound crystallizes in the orthorhombic space group Cccm, with unit cell parameters a = 6.3892(4), b = 19.6003(15) and c = 10.3570(6) Å, V = 1297.01(15) ų, and Z = 4. Two symmetrically independent octahedra in the structure are jointly occupied by Ni and Al atoms. By sharing oxygen vertices, these octahedra form intercrossed chains stretched in the [101] and [01] directions to build two-layered slabs perpendicular to the b axis. Phosphate tetrahedra strengthen the slabs by sharing most vertices and edges with the octahedra. One unshared vertex of each P1O₄ polyhedron is directed in the area between the slabs, which includes strongly disordered Na atoms. The title crystal structure is discussed as being derived from the α-CrPO₄ archetype, with a framework conversion from triperiodic to biperiodic slabs. The theoretical specific capacity of Na_4.3Ni_1.3Al_1.7(PO₄)₄ for the deintercalation of all sodium ions, equal to 140 mA h g⁻¹, and the value of the migration barrier of about 0.5 eV in all three directions allow considering the compound as a potential material for the positive electrode of sodium-ion batteries. Magnetically, Na_4.3Ni_1.3Al_1.7(PO₄)₄ consists of isolated Ni²⁺ ions and pairs of these ions coupled into ferromagnetic dimers. The exchange interactions between these entities are weak, which prevents the long-range magnetic order down to 2 K. The Schottky-type anomaly in the specific heat at low temperatures points to the energy gap between ferromagnetic ground state and antiferromagnetic excited state within the dimers.