Synthesis and characterization of AsO[(W,Mo)O3]13, a new (6)-intergrowth tungsten bronze (ITB)

Abstract

Crystals of a new arsenic tungsten-molybdenum oxide, AsO[(W,Mo)O3]13, have been grown through vapour phase transport at high temperature (1123 ​K). The crystal structure was investigated by combining electron and single-crystal X-ray diffraction techniques, electron microprobe chemical analysis and X-ray absorption spectroscopy. Single-crystal XRD yielded the orthorhombic unit-cell parameters a ​= ​25.0895, b ​= ​7.3061, c ​= ​3.9089 ​Å. Although electron diffraction indicated a doubled translation along [001], superstructure reflections were undetectable with X-ray diffraction so that only the average structure (sub-cell) was determined. The most reliable structural model was obtained in the P222 space group (R1 ​= ​6.96%). It exhibits an octahedral framework which forms six-octahedra thick perovskite slabs (PTB) alternated to single hexagonal tungsten bronze (HTB) modules, indicating that the compound corresponds to a (6)-ITB phase. As3+ was found to be disordered on two off-centred, symmetry-related sites into the hexagonal channels with the typical pyramidal AsO3 geometry. This is in agreement with the results of the extended X-ray absorption fine structure that indicate an As–O bond distance of 1.77 ​Å. The ordered distribution of As within the tunnels, likely related to the doubling of periodicity along [001], could not be determined.