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

Abstract
A solution to the phase problem is described which gives the phase invariant Ψ=2φhφg 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=(002¯) and g=(004¯) we find Ψ=54.4°+0.9°. The error in the derived (002¯) x-ray structure-factor phase is 0.069°. The method is accurate enough to provide information on the bonding charge distribution in noncentric crystals.

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