Unique phase determination on the basis of Bijvoet ratios by means of resonant neutron scattering

A new two-wavelength Bijvoet-pair method is proposed, which allows unique experimental determination of anomalous phases without absorption correction and scaling of the Bragg intensities, if the resonant atom structure is known. The method consists of measuring Bijvoet ratios at two wavelengths λ₁ and λ₂, for which the real and imaginary dispersion terms in the anomalous scattering amplitude fulfil the conditions: b′_A(λ₁) = -b′_A(λ₂) and b′′_A(λ₁) = b′′_A(λ₂). These conditions can be fulfilled with resonant neutron scattering by nuclei like ¹¹³Cd, ¹⁴⁹Sm or ¹⁵⁷Gd, but not with resonant X-ray or Mössbauer γ-ray scattering. A first application of the method to experimental phase determination for a small structure, Cd-histidine, is described.