Accurate structure-factor phase determination by electron diffraction in noncentrosymmetric crystals

A solution to the phase problem is described which gives the phase invariant =2h-g for electron structure factors with phases h and g 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 =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.