Nuclear and magnetic structures and magnetic properties of synthetic brochantite, Cu4(OH)6SO4

Abstract

Cu₄(OH)₆SO₄ (1) and Cu₄(OD)₆SO₄ (2) were obtained by hydrothermal syntheses from copper sulfate and sodium hydroxide in H₂O and D₂O, respectively. They crystallize in the monoclinic system, space group P2₁/a (14), a = 13.1206(5), b = 9.8551(3), c = 6.0295(2) Å, β = 103.432(3)°, V = 758.3(1) ų, Z = 4 and a = 13.1187(5), b = 9.8552(3), c = 6.0293(2) Å, β = 103.410(3)°, V = 758.3(1) ų, Z = 4, respectively. They are iso-structural to the mineral brochantite and consist of double chains of edge-sharing copper octahedra that are connected to one another by corners to form corrugated planes along bc; these planes are in-turn bridged by the unprecedented µ⁷-sulfate tetrahedra to give a 3D-structure. All the hydrogen atoms were precisely located from refinement of the neutron powder diffraction data of the deuterated sample. Magnetic susceptibility data reveal a low-dimensional behavior at high temperature and the presence of both ferromagnetic and antiferromagnetic super-exchanges resulting in a 3D long-range antiferromagnetic ordering at 7.5 K accompanied by a small canting of the moments. The transition is confirmed by a λ-peak in the specific heat. The magnetic structure at 1.4 K shows the moments are oriented perpendicular to the corrugated planes with alternation along ±a for neighboring chains within the double chains. The enhanced incoherent scattering at low-angle suggests the existence of short-range ferromagnetic clusters.