A new approach for the calculation of diffraction profiles in strained crystals is developed, based on the visual concepts of the dispersion surface and Poynting vectors. By this approach, analytical expressions have been obtained for diffraction profiles for the case of a constant strain gradient without, as well as with, ultrasonic excitation. Calculations of acoustically induced modifications in diffraction spectra explain in detail the anomalous dependence of integrated intensity on ultrasound amplitude, a dependence that was recently found in the Laue scattering geometry.

Surface acoustic waves were combined with double-crystal X-ray diffractometry in order to study strained silicon crystals. A narrowing effect in the rocking curves was observed owing to the ultrasound-induced modification of the X-ray trajectories. It is shown that the narrowing effect is strongly related to the lattice strain and can be used to increase the strain sensitivity of the X-ray diffraction technique.

The X-ray optics in a strained single-crystal under ultrasonic excitation are considered. The anomalous behavior of diffraction intensity, depending on sound amplitude, is analyzed. The interference of X-rays moving along different trajectories is demonstrated, which leads to a new type of Pendellösung effect, depending on the strain gradient and sound frequency. Experimental data agree with the theoretical predictions.

A study of X-ray diffraction in single silicon and quartz crystals in a strong ultrasonic field has been carried out. The dependence of diffraction intensity I(w) on sound amplitude w has been obtained right up to the kinematical limit I_k = I(∞). It is shown that the ratio η = I(∞)/I(0) depends on the crystal quality. Experimental data for different reflections, sound frequencies and X-ray wavelengths are compared with the model calculations for several lattice-distortion mechanisms.