Dinuclear oxofluorometallates as a new structural type of d~ 0 transition metal oxofluoride compound

摘要

Five isomorphous d~ 0 transition metal oxofluoride compounds A 3[M 2O x F11 - 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 M 2 X 11 (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 M 2O4F7 (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 ?, which is the [101] diagonal of the orthorhombic unit cell. The other four trigonal crystals are merohedral twins.