Some Anomalous Absorption Effects on the Shape of Single-Crystal Rocking Curves

Abstract

For most absorbing crystals set in the transmission Laue geometry, the forward-diffracted x-ray-beam rocking curve consists of a peak in intensity at angles slightly less than the Bragg angle ${\theta }_B$ followed by a dip in intensity as the crystal is rocked toward increasing $\theta$. This shape is due to the less-than-normal absorption (i.e., anomalous transmission) of $\alpha$-branch waves of the dispersion surface, which are dominant for angles less than ${\theta }_B$, and the more-than-normal absorption of $\beta$-branch waves, which are dominant at angles larger than ${\theta }_B$. However, it is possible to choose a crystal where $\beta$-branch waves undergo anomalous transmission. In this event, the peak in intensity will be found at angles slightly larger than ${\theta }_B$. Experimental verification of this phenomenon is presented, using the particular case of (200) reflections from InAs or GaSb, using Mo $K_{\alpha }$ x rays. The InAs shows the rocking-curve profile inversion, while structurally similar GaSb shows normal behavior. The difference between InAs and GaSb is due to the difference in contributions of the imaginary parts of the atomic scattering factors to the structure factor.