Ionothermal Synthesis, Crystal Structure, and Magnetic Study of Co₂PO₄OH Isostructural with Caminite

A new framework cobalt­(II) hydroxyl phosphate, Co₂PO₄OH, was prepared by ionothermal synthesis using 1-butyl-4-methyl-pyridinium hexafluorophosphate as the ionic liquid. As the formation of Co₂PO₄F competes in the synthesis, the synthesis conditions have to be judiciously chosen to obtain well-crystallized, single phase Co₂PO₄OH. Single-crystal X-ray diffraction analyses reveal Co₂PO₄OH crystallizes with space group I4₁/amd (a = b = 5.2713(7) Å, c = 12.907(3) Å, V = 358.63(10) ų, and Z = 4). Astonishingly, it does not crystallize isotypically with Co₂PO₄F but rather isotypically with the hydroxyl minerals caminite Mg_1.33[SO₄­(OH)_0.66(H₂O)_0.33] and lipscombite Fe_2–yPO₄(OH) (0 ≤ y ≤ 2/3). Phosphate tetrahedra groups interconnect four rod-packed face-sharing _∞¹{CoO_6/2} octahedra chains to form a three-dimensional framework structure. The compound Co₂PO₄OH was further characterized by powder X-ray diffraction, Fourier transform–infrared, and ultraviolet–visible spectroscopy, confirming the discussed structure. The magnetic measurement reveals that Co₂PO₄OH undergoes a magnetic transition and presents at low temperatures a canted antiferromagnetic spin order in the ground state.