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

• Published in: Inorganic chemistry Vol. 56; no. 18; pp. 10950 - 10961
• Main Authors: Ludwig, Jennifer, Geprägs, Stephan, Nordlund, Dennis, Doeff, Marca M, Nilges, Tom
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 18.09.2017; American Chemical Society (ACS)
• Subjects: INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.7b01152

A new metastable phase, featuring a lithium-stabilized mixed-valence cobalt­(II,III) hydroxide phosphate framework, Co11.0(1)Li1.0(2)[(OH)5O]­[(PO3OH)­(PO4)5], corresponding to the simplified composition Co1.84(2)Li0.16(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) Å3, 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.