A novel cobalt sodium phosphate hydroxide with the ellenbergerite topology: crystal structure and physical properties

Abstract

The novel phase Na_2−xCo₆(OH)₃[HPO₄][H_x/3PO₄]₃ (x ≈ 1.1) was prepared by hydrothermal synthesis at 553 K. Its crystal structure was determined using single-crystal X-ray diffraction data and refined against F² to R = 0.052, including positions of all hydrogen atoms. The compound crystallizes in the hexagonal space group P6₃mc, with unit-cell parameters a = 12.630(3) Å, c = 5.017(1) Å, V = 693.1(3) ų, and Z = 2. The crystal structure is based on a 3D framework built from CoO₆ octahedra and PO₄ tetrahedra. Channels in the [001] direction accommodate columns of Na-centered octahedra sharing faces. The compound is a new structural representative of the topology shown by aluminosilicate mineral ellenbergerite and its numerous natural and synthetic varieties. Magnetic susceptibility measurements revealed a strong antiferromagnetic interaction and magnetic transition to low temperature spin-canted phase at T_N = 44 K. The physical properties of the title compound are found to be very similar to those of the structurally related arsenate Co_1−xCo₆(OH)₃[H_2x/3AsO₄]₃[HAsO₄] and vanadate Co₇(OH)₂(H₂O)[VO₄]₄.