Absolute X-Ray Reflectivities of Single Crystals of Calcite, Rocksalt, Rochelle Salt and Barite

With a two-crystal spectrometer absolute x-ray reflectivity measurements have been performed upon four different crystals. In the case of a large calcite crystal of uniform appearance, considered a good spectrometer crystal, with $K\alpha$ radiation of silver it is found that the theoretical values of percent reflection, integrated reflection, and rocking curve width calculated by Allison on the basis of the Darwin-Prins theory of reflection by a perfect absorbing crystal are not attained, the experimental and theoretical quantities being related by a factor not far from two in each case. Rocksalt crystals from a large exhibition specimen from Russia were found to be much more perfect in structure than any rocksalt specimens heretofore reported in the literature of x-ray spectrometry. Rocksalt reflectivity data for Ag $K\alpha$ are given. Artificial Rochelle salt crystals are found to possess rocking curve widths as narrow as those of high-grade calcite, indicating a very uniform structure; at the same time the reflections are relatively weak. Rochelle salt reflectivity data for Ag $K\alpha$ are given. Percent reflection, rocking curve width and integrated reflection of barite were determined for eleven wave-lengths, and scattering factors were calculated. The variation of the scattering factor with wave-length in the neighborhood of the barium $K$ limit is similar to the variation previously found by Wyckoff and by Glocker and Schäfer for lighter elements except that Wyckoff's maximum does not appear.