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

Abstract

Images of pale yellow crystals of 4Nb205. 9WO 3, 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-Mstrings 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 '6Nb205. 1 IWO 3' phase. The resolving power of the present microscope is discussed on the basis of the analysis of the chromatic aberration.

A New Niobium Tungsten Oxide as a Result of anin SituReaction in a Gas Reaction Cell Microscope

“…Figure 1a presents the HRTEM micrograph along [001] of a crystal fragment (designated as crystal A in the following) of the starting material Nb W O . This image was recorded from a thin crystal region at close to the Scherzer defocus of the instrument so that the positions of the heavy atoms are recognizable a dark contrasts (15). This interpretation is confirmed by the simulation (inset in Fig.…”

Superstructure and Twinning in the Tetragonal Tungsten Bronze-Type Phase Nb7W10O47

“…The observed twinning can be described by a rotation of 903 around the crystallographic c axis, and, in addition, the inversion twins of the main and the rotated individuum are present, leading to a fourling. The main individuum and the rotated component were re"ned as free parameters to values of 0.727(37) and 0.069 (14) of the total volume. The volumes of the inversion twins are present with fractions of 0.087(9) and 0.117 (14), respectively.…”

“…The main individuum and the rotated component were re"ned as free parameters to values of 0.727(37) and 0.069 (14) of the total volume. The volumes of the inversion twins are present with fractions of 0.087(9) and 0.117 (14), respectively.…”

The Crystal Chemistry of the Higher Tungsten Oxides

“…52 " 54 The structure of this phase is shown in Fig. The widest range of such phases have been reported from the Nb 2 0 5 -W0 3 and Ta 2 0 5 -W0 3 systems for compositions between approximately M0 3 and Μ0 2 .…”