The multiple-diffraction effect in accurate structure-factor measurements of PtP₂ crystals

Accurate X-ray intensities of a PtP₂ crystal were measured with the multiple-diffraction effect avoided by using the ψ rotation of the crystal around the scattering vector. The results were compared with those of a measurement made without avoiding the effect. The extinction parameters were more isotropic and the peaks of the deformation density were significantly smaller when multiple diffraction was avoided. In the measurement made without avoiding the effect, the number of structure factors affected by more than 1% was 403 from 936 reflections measured. Of 272 strong reflections with sin θ/λ < 0.6 Å⁻¹ and F_obs > 200, 27 reflections were affected by more than 1%. These facts, as well as the smaller R values, indicate that the multiple-diffraction effect cannot be neglected in electron-density studies of crystals including heavy atoms. To assess the intensity fluctuation calculated in the present study, the intensities of 200 reflections were measured at intervals of 0.5° from 0 to 180° in ψ and compared with the calculated values. The variations of the measured and calculated intensities with ψ correspond to each other, which indicates that the present method can be used effectively to identify the reflections affected significantly by multiple diffraction.