Abstract
Single crystals of NH 4CoAlF 6 (I) are grown by hydrofluorothermal synthesis under 3 kbar pressure. A single-crystal, X-ray structural determination showed the compound is orthorhombic and adopts a pyrochlore-related RbNiCrF 6-type structure with an ordered arrangement of Co and Al in the octahedral sites. The observed orthorhombic symmetry is a direct consequence of the cation ordering in the octahedral sites of the pyrochlore lattice. However, NH 4CoAlF 6 (II) synthesized by thermal decomposition of hexaamminecobalt hexafluoroaluminate, (NH 3) 6CoAlF 6, adopts a cubic structure and the structural refinement using high-resolution synchrotron X-ray diffraction data showed this phase also has a structure similar to RbNiCrF 6 but with a disordering of Co and Al atoms in the octahedral sites. The cubic NH 4CoAlF 6 phase (II) is metastable and can be converted to the orthorhombic phase (I) by prolonged heating at 350 °C in a sealed platinum tube. Stabilization of an ordered structure for NH 4CoAlF 6 is apparently due to the significant difference in the ionic radii of octahedrally coordinated Co 2+ and A1 3+ cations.
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