Refinement of the defect structure of `GeNb₉O₂₅' by high-resolution electron microscopy

Abstract

It has been previously established that calculations of intensity distributions in high-resolution electronmicroscope images of crystal structures can reproduce the experimental observations. The use of comparisons Of observed and computed images for structure analysis has been extended for the first time to the refinement of a defect structure in the case of crystals of nominal composition 'GeNb902f. The results show that previously proposed models for the disordered atom configurations around 'tetrahedral' sites are not completely correct. A new model is proposed which shows good agreement between computed and observed images. This model suggests an interpretation for observations on other related structures in terms of a new type of defect.

Computed electron microscope images of atomic defects in f.c.c. metals

“…The basis of the present calculations is the Cowley & Moodie (1957)multislice n-beam dynamical diffraction theory, adapted for numerical analysis by computer as discussed by Goodman & Moodie (1974). The computer program used was written by Skarnulis (1975) and has been modified and expanded by M. A. O'Keefe. Results using this program for calculating perfect crystal images have been presented by Skarnulis, Iijima & Cowley (1976) and O'Keefe & Anstis (in preparation).…”

On the thickness periodicity of atomic-resolution images of dislocation cores

“…Many-beam crystal-structure images recorded in the zone-axis orientation are commonly interpreted as showing a projection of the crystal structure taken in the beam direction (Skarnulis et al 1976, Lynch, Moodie and O'Keefe 1975). We first consider the most favourable choice of projection direction from which to obtain atomic positions around a dislocation core.…”

“…These defects frequently control the thermal, chemical, mechanical and electronic properties of solids. At present the smaller unit cell of most simple metals prevents the routine application of the successful methods of structure imaging used in mineralogy and solid state chemistry (see, for example, Skarnulis, Iijima and Cowley 1976). The 3.4 A point resolution of the current generation of transmission electron microscopes remains insufficient t o resolve the atomic structure of defects in metals, dcspite the impressive lattice resolution obtained from perfect crystals in the publicat.ions cited above.…”

Achievement of atomic resolution electron microscopy

“…The task of obtaining a satisfactory match between the experimental micrographs and simulated images for alternative defect models is then much easier. For a structure determination of the complex inorganic oxide "GeNb9025'' the similarity in appearance of experimental and simulated images was used (Skarnulis et al, 1976).…”