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 <i>P</i>31<i>m</i> (<i>a =</i> 11.2533(4) Å, <i>c =</i> 4.9940(2) Å, <i>V =</i> 547.70(3) ų, <i>Z =</i> 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 <i>P</i>31<i>m</i>-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 <i>T</i> = 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