n-Beam lattice images. I. Experimental and computed images from W₄Nb₂₆O₇₇

A series of through-focus lattice images has been obtained with an electron microscope for single crystals of W₄Nb₂₆O₇₇ set at an orientation to give mainly a systematic set of 00l reflexions. Dynamical n-beam computations (n = 435) have been made for a crystal in this orientation to determine the values of the amplitudes and phases of the diffracted beams. Subsequently, the fringe profiles for lattice images have been computed for various thicknesses of crystal and defects of focus. When the experimental factors, including instrument aberrations, are taken into account, good agreement between computed and observed fringes is obtained. An earlier assumption of direct correlation of features in underfocused images with structure in the crystal, is found to be valid, at least for this compound.