Dinuclear oxofluorometallates as a new structural type of d0 transition metal oxofluoride compound

摘要

Five isomorphous d0 transition metal oxofluoride compounds A₃[M₂O_xF_11???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??, which is the [101] diagonal of the orthorhombic unit cell. The other four trigonal crystals are merohedral twins.