Accurate Structure-Factor Phase Determination by Electron Diffraction in Noncentrosymmetric Crystals

A solution to the phase problem is described which gives the phase invariant Ψ=2φₕ-φg for electron structure factors with phases φₕ and φg in noncentric crystals. The method exploits the high-voltage dependence of a minimum in convergent-beam transmission-electron-diffraction patterns in the systematics three-beam geometry. For CdS, with h=(002̄) and g=(004̄) we find Ψ=54.4^∘+0.9^∘. The error in the derived (002̄) x-ray structure-factor phase is 0.069^∘{}. The method is accurate enough to provide information on the bonding charge distribution in noncentric crystals.