The structure of multiple twinned CsMgInF6 and its relationship to other pyrochlore derived fluorides

The pyrochlore derived fluoride structures with general composition AM(2+)M(3+)F(6) crystallize in space groups which are in group-subgroup relationship to Fd $(3) over bar $m, the space group of the archetype pyrochlore structure. All orthorhombic representatives investigated so far show an ordered M2+/M3+ distribution over two symmetrically independent octahedral sites. CsMgInF6 crystallizes in space group puma with lattice parameters a = 7.5285(1), b = 7.5285(1) and c = 10.6459(1) angstrom. The investigated crystal was a second degree twin, where the independent twin elements are a combination of the three- and fourfold rotation axes of the archetype cubic lattice. The pseudosymmetry of the compound with respect to space group Imma is extremely high and a reliable decision about the correct symmetry was only possible when inspecting partial agreement factors for the weak reflections h + k + l = 2n + 1 only. With respect to the other orthorhombic pyrochlore derived ternary fluorides, the present compound differs in the fact that the M2+/M3+ ions are completely disordered in CsMgInF6.