Study of modulated structures by transmission electron microscopy

Abstract

We review the application of transmission electron microscopy to the study of displacively modulated transition metal chalcogenides at low temperatures. The weak scattering (kinematical diffraction) from these structures is itself quite complicated, and some of the salient features are described. Multiple scattering (dynamical diffraction) adds to the complexity of interpreting electron diffraction patterns. Several different multiple scattering effects are identified and then a method is described, the satellite Tanaka method, which proves particularly effective in tackling some of these problems. Useful symmetry information can be obtained by convergent-beam electron diffraction from commensurately modulated structures although its application to incommensurate phases requires further investigation. In the case of commensurately modulated 2H TaSe2 the atomic displacements from their high-temperature equilibrium positions has been determined by a novel method. Two distinct methods of imaging have been used; satellite dark field microscopy and high-resolution microscopy. Some interesting results have been obtained by the former technique, but problems of image interpretation can arise. High-resolution microscopy of these phases provides a difficult challenge and requires further effort in the future. Show more