The structure of multiple twinned CsMgInF6and its relationship to other pyrochlore derived fluorides

Abstract

AbstractThe pyrochlore derived fluoride structures with general composition AM2+M3+F6crystallize in space groups which are in group-subgroup relationship toFd-3m, 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. CsMgInF6crystallizes in space groupPnmawith lattice parametersa= 7.5285(1),b= 7.5285(1) andc= 10.6459(1) Å. 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 groupImmais extremely high and a reliable decision about the correct symmetry was only possible when inspecting partial agreement factors for the weak reflectionsh+k+l= 2n+ 1 only. With respect to the other orthorhombic pyrochlore derived ter nary fluorides, the present compound differs in the fact that the M2+/M3+ions are completely disordered in CsMgInF6.