Invariant-Phase Information of X-Ray Structure Factors in the Two-Beam Bragg Intensity near a Three-Beam Point

Dynamical theory yields a simple expression for the integrated intensity $I_{0H}$ of a symmetric two-beam Bragg reflection versus azimuthal angle around $\overrightarrow{\mathrm{H}}$, perturbed to first order as another reciprocal lattice point $\overrightarrow{\mathrm{L}}$ approaches the Ewald sphere. Around the three-beam point $I_{0H}$ shows pronounced asymmetry sensitive to the magnitude and phase of the product of all three structure factors. The fully quantitative description allows optimization of the conditions for determination of that phase. It applies regardless of inversion symmetry.