1 research outputs found
Co<sub>11</sub>Li[(OH)<sub>5</sub>O][(PO<sub>3</sub>OH)(PO<sub>4</sub>)<sub>5</sub>], a Lithium-Stabilized, Mixed-Valent Cobalt(II,III) Hydroxide Phosphate Framework
A new metastable
phase, featuring a lithium-stabilized mixed-valence cobaltÂ(II,III)
hydroxide phosphate framework, Co<sub>11.0(1)</sub>Li<sub>1.0(2)</sub>[(OH)<sub>5</sub>O]Â[(PO<sub>3</sub>OH)Â(PO<sub>4</sub>)<sub>5</sub>], corresponding to the simplified composition Co<sub>1.84(2)</sub>Li<sub>0.16(3)</sub>Â(OH)ÂPO<sub>4</sub>, is prepared
by hydrothermal synthesis. Because the pH-dependent formation of other
phases such as Co<sub>3</sub>(OH)<sub>2</sub>Â(PO<sub>3</sub>OH)<sub>2</sub> and olivine-type LiCoPO<sub>4</sub> 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) Ã…, <i>c =</i> 4.9940(2) Ã…, <i>V =</i> 547.70(3) Ã…<sup>3</sup>, <i>Z =</i> 1). The three-dimensional network structure
is characterized by partially Li-substituted, octahedral [M<sub>2</sub>O<sub>8</sub>(OH)] (M = Co, Li) dimer units which form double chains
that run along the [001] direction and are connected by [PO<sub>4</sub>] and [PO<sub>3</sub>(OH)] tetrahedra. Because no Li-free <i>P</i>31<i>m</i>-type Co<sub>2</sub>(OH)ÂPO<sub>4</sub> 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, Co<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>, and olivine-type LiCoPO<sub>4</sub> with release of water and oxygen