Zur Kristallstruktur von Pyrochloren: Mössbauer‐Spektrum von orthorhombischem CsFe2F6 und röntgenographische Einkristall‐Untersuchungen an den kubischen Verbindungen CsMgGaF6, CsMIIVIIIF6 (MII = Mn, Zn), CsMIIFeIIIF6 (MII = Mn, Cu, Zn) und Cs4Cu5V3O2F19

On the Crystal Structure of Pyrochlores: Mössbauer Spectra of Orthorhombic CsFe2F6 and X‐Ray Single Crystal Studies of the Cubic Compounds CsMgGaF6, CsMIIVIIIF6 (MII = Mn, Zn), CsMIIFeIIIF6 (MII = Mn, Cu, Zn), and Cs4Cu5V3O2F19 Orthorhombic CsFe2F6 (a = 749.1(3), b = 723.8(3), c = 1043.1(6) pm; V = 565.6(9) Å3, Z = 4, Imma ?) was prepared with strongly 57Fe doped FeF3 and its Mössbauer spectra measured. It is in accordance with the spectra of a natural 57Fe abundance sample, also with respect to the intensity ratio FeII:FeIII = 1:1. For the formation of an ordered pyrochlore structure in spite of quenching thus an electronically induced phase transition should be responsible, explaining the MII/MIII order observed in all mixed valence pyrochlores AIMIIMIIIF6. By contrast, single crystals of CsMgGaF6 (a = 1021.6(1) pm), CsMnVF6 (a = 1058.9(1) pm), CsMnFeF6 (a = 1054.6(1) pm), CsZnVF6 (a = 1041.5(1) pm, CsZnFeF6 (a = 1042.1(1) pm) and CsCuFeF6 (a = 1037.7(2) pm), obtained by solid state reaction and slow cooling, all exhibit the cubic pyrochlore structure of the RbNiCrF6 type (Z = 8, ${\rm Fd}{\bar 3}\rm m$), in which the cations MII/MIII are disordered in position (16c). An oxidfluoride of approximate composition Cs4Cu5V3O2F19 (a = 1022.2(1) pm) has the same structure, but in addition it shows a disordered occupation of the position (48f) by lack of about 1/8 of anions. Structural relations and distances for the cubic crystal structures refined (R between 0.02 and 0.06) are discussed.