Large-angle convergent-beam electron diffraction; a simple technique for the study of modulated structures with application to V₂D

With a highly convergent electron beam entering a small crystal area, the Bragg condition may be fulfilled simultaneously for many diffraction orders along a reciprocal-lattice direction, and reflections with (sin θ)/λ > 2 Å⁻¹ are accessible. Considerations based on the dynamical theory of electron diffraction suggest that for Bragg reflections with (sin θ)/λ > 1 Å⁻¹, 100 keV electrons can be treated quasi-kinematically when the crystal thickness is less than 1000 Å, and this is supported by experiments. This simple approach has been used to determine the D-induced static displacement modulation δ of V along the a axis (monoclinic indexing) in the β phase of V₂D. From visual inspection of the Bragg intensities for large diffraction orders, up to (sin θ)/λ = 2.35 Å⁻¹, it is concluded that δ = 0.070 (5) Å.