Acta Cryst. (2012). B68, 602-609 [ doi:10.1107/S0108768112042577 ]
Abstract: Five isomorphous d0 transition metal oxofluoride compounds A3[M2OxF11 - x]·(AF)0.333 (A = K, Rb, NH4; M = Nb, Mo, W; x = 2, 4) have been synthesized from acid fluoride solutions, and their crystal structures have been determined by single-crystal X-ray diffraction. The basic structural building units are dinuclear M2X11 (dimers) formed from NbOF5 or Mo(W)O2F4 octahedra connected by the fluorine bridging atom. In the Nb2O2F9 dimer, the O atoms occupy apical corners. In the M2O4F7 (M = Mo, W) dimers two O atoms are also apically placed, whereas the other two O atoms are statistically disordered in equatorial planes. The arrangement of dimers is so that the hexagonal tunnels containing `free' fluoride ions are formed. During the irradiation process the orthorhombic structure of K3Nb2O2F9·(KF)0.333 transforms into a pseudo-trigonal one with a = 23.15 Å, which is the  diagonal of the orthorhombic unit cell. The other four trigonal crystals are merohedral twins.
Keywords: dinuclear oxofluorometallates; vibrational spectra; bridging fluorine atoms; merohedral twins.
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