The crystal structure of 4Nb₂O₅.9WO₃ studied by 1 MV high-resolution electron microscopy

Images of pale yellow crystals of 4Nb₂O₅. 9WO₃, obtained with a 1 MV high-resolution electron microscope revealed twinned domains of a tetragonal tungsten bronze structure with a superlattice of 3 x 1 subcells. Comparison with computer calculations suggests that the cations filling the pentagonal tunnels include both Nb and W. Crystals darkened due to reduction on longer heating included no domains and were sensitive to electron irradiation; cations were knocked on from the filled to the vacant pentagonal tunnels. This suggests that some oxygens are released from the -M-O-M-O-M- strings in the tunnels on reduction to weaken the chemical bonding. The number of deficient oxygens is known from the weight gain on oxidizing the crystal. Some additional experiments reveal that there is no '6Nb₂O₅. 1 IWO₃' phase. The resolving power of the present microscope is discussed on the basis of the analysis of the chromatic aberration.