n-Beam lattice images. III. Upper limits of ionicity in W₄Nb₂₆O₇₇

The scattering factor for electrons is sensitive to differences in atomic bonding at low values of sin θ/λ. These differences will influence the amplitudes and phases of low-order beams of electrons diffracted from crystals with large unit cells. The experimental intensities of these beams, and contrast in the corresponding lattice images, can be used to derive upper limits for the charges on constituent ions, provided the associated n-beam dynamical calculations are carried out with sufficient precision. It is shown that, in W₄Nb₂₆O₇₇, the atoms can be only partially ionized, and the bonding must have some covalent character.