Accuracy in the Use of Electron-Diffraction Spot Patterns for Determining Crystal Orientations

Abstract

It has long been known that transmission electron-diffraction patterns appropriate to low-index zone axes in thin crystals exist over several degrees of tilt from the exact orientations. In this work the magnitude of the errors in determining crystal orientation from spot patterns has been investigated using precisely oriented crystals prepared by vapor deposition and an electron microscope equipped with a goniometer specimen stage. It was observed that patterns of low-index orientations existed over ranges as high as 20° tilt, and were insensitive to the size of the area diffracted. Tilt ranges were observed to increase noticeably when the foils were plastically deformed. However, orientations accurate to 0.5° may be determined by weighting the pattern according to the intensities of the diffraction spots. Elastic bending of the foils and ``polygonization'' bending due to dislocations help to explain why the relrods are so much ``longer'' than predicted on the basis of relaxation of the Laue conditions, using kinematic theory.