NH4CoAlF6: A pyrochlore-related phase existing in both ordered and disordered forms

Single crystals of NH4CoAlF6 (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 RbNiCrF6-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, NH4CoAlF6 (II) synthesized by thermal decomposition of hexaamminecobalt hexafluoroaluminate, (NH3)6CoAlF6, 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 RbNiCrF6 but with a disordering of Co and Al atoms in the octahedral sites. The cubic NH4CoAlF6 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 NH4CoAlF6 is apparently due to the significant difference in the ionic radii of octahedrally coordinated Co2+ and A13+ cations.