Co₁₁Li[(OH)₅O][(PO₃OH)(PO₄)₅], a Lithium-Stabilized, Mixed-Valent Cobalt(II,III) Hydroxide Phosphate Framework

A new metastable phase, featuring a lithium-stabilized mixed-valence cobalt(II,III) hydroxide phosphate framework, Co_11.0(1)Li_1.0(2)[(OH)₅O][(PO₃OH)(PO₄)₅], corresponding to the simplified composition Co_1.84(2)Li_0.16(3)(OH)PO₄, is prepared by hydrothermal synthesis. Because the pH-dependent formation of other phases such as Co₃(OH)₂(PO₃OH)₂ and olivine-type LiCoPO₄ competes in the process, a pH value of 5.0 is crucial for obtaining a single-phase material. The crystals with dimensions of 15 μm × 30 μm exhibit a unique elongated triangular pyramid morphology with a lamellar fine structure. Powder X-ray diffraction experiments reveal that the phase is isostructural with the natural phosphate minerals holtedahlite and satterlyite, and crystallizes in the trigonal space group P31m (a = 11.2533(4) Å, c = 4.9940(2) Å, V = 547.70(3) ų, Z = 1). The three-dimensional network structure is characterized by partially Li-substituted, octahedral [M₂O₈(OH)] (M = Co, Li) dimer units which form double chains that run along the [001] direction and are connected by [PO₄] and [PO₃(OH)] tetrahedra. Because no Li-free P31m-type Co₂(OH)PO₄ phase could be prepared, it can be assumed that the Li ions are crucial for the stabilization of the framework. Co L-edge X-ray absorption spectroscopy demonstrates that the cobalt ions adopt the oxidation states +2 and +3 and hence provides further evidence for the incorporation of Li in the charge-balanced framework. The presence of three independent hydroxyl groups is further confirmed by infrared spectroscopy. Magnetization measurements imply a paramagnetic to antiferromagnetic transition at around T = 25 K as well as a second transition at around 9-12 K with a ferromagnetic component below this temperature. The metastable character of the phase is demonstrated by thermogravimetric analysis and differential scanning calorimetry, which above 558 °C reveal a two-step decomposition to CoO, Co₃(PO₄)₂, and olivine-type LiCoPO₄ with release of water and oxygen.