Synthesis and crystal structure of a series of stoichiometric (n)-ITB molybdenum-bronze oxides containing trivalent arsenic

摘要

A series of six new single crystals of fully stoichiometric As3+-bearing Mo-oxides and partially W-substituted Mo-oxides with formula AsmO(Mo1-xWxO3) p (m = 1, 2; p = 5, 7, 9, 10 and 11 and 0 <= x <= 0.6) was successfully grown using vapor-phase transport in vacuo. The crystal structures were determined using single-crystal X-ray diffraction data. All these compounds exhibit acentric orthorhombic symmetry with Z = 2, and belong to the socalled (n)-ITB (intergrowth tungsten bronzes) family, with n = 2, 3, 4 and 5. The six (n)-ITB phases have the following formulae: (2)-AsMo5O16 (Pm2a), (2)-As2Mo10O31 (Pma2), (3)-AsMo7O22 (Pmn2(1)), (3)-As(Mo5.53W1.47)O-22 (Pmn2(1)), (4)-As(Mo4.33W4.67)O-28 (Pm2a) and (5)-As(W6.63Mo4.37)O-34 (Pmn2(1)). Their structures consist of vertex-sharing MO6 octahedral units (with M either Mo or Mo/W) connected so as to form three-dimensional frameworks. Such frameworks consist of perovskite tungsten bronzes (PTB) type slabs, from 2- to 5-octahedra wide, intergrown with single hexagonal tungsten bronzes (HTB) type slabs, stacked up to form pseudo-hexagonal tunnels along the a-axis. As3+ and additional oxygen atoms are located in off-center positions inside the tunnels, forming As-O bonds with peculiar arrangements. In particular, we obtained the first examples of structures where, besides the usual AsO3E distorted pyramidal geometry, As3+ adopts AsO4E coordination with a seesaw configuration.