The existence of non-Laue maxima in the diffraction of x-rays was first recognized in 1938 by Wadlund for a continuous spectrum of x-rays and by Laval for monochromatic x-rays. A brief description of Laval's exhaustive work is given. The main points of the theories of Zachariasen, of Raman, Nilakantan and Nath, and of Preston, Bragg and Jauncey are also described. In the authors' experiments $\mathrm{Cu}K\alpha$, $\mathrm{Cu}K\beta$, or $\mathrm{Mo}K\alpha$ x-rays with a weak background of continuous radiation were diffracted from an etched cleavage face of a thick rocksalt crystal. With the crystal face set to give the 200 reflection of $\mathrm{Cu}K{\alpha }_1$ a total divergence of 1.6′ was found in the reflected beam, indicating very little warping in the layers near the crystal surface. With the crystal face turned by steps of 1° from a glancing angle of 45°07′ to 28°07′ the positions of the non-Laue maxima associated with the 400 and 620 Bragg reflections were found. The non-Laue spots are called associated Bragg spots. All photographs showed a shift of the maximum intensity of the associated Bragg spot from the position of the corresponding Bragg spectrum line which is greater than zero. This result differs from that of Raman and Nilakantan, who report zero shift with rocksalt. The observed shift is always less than Zachariasen's theoretical value for a given $\Delta$, the angle by which the crystal is set away from the position for a Bragg reflection. Possibly for $\Delta <1^{\circ }$ the Zachariasen formula holds. From the value of $\Delta$ at which the associated Bragg spots disappear the shortest effective wave-lengths of the elastic waves concerned are about 13A to 17A. These are between 2 and 3 times the minimum wave-length in Debye's specific heat theory. On the Preston-Bragg-Jauncey view a shift less than the Zachariasen value implies that the groups of atoms which give rise to the associate Bragg spots are considerably larger than an atom plus its 12 nearest neighbors as supposed by Preston.

• Received 1 February 1941

DOI:https://doi.org/10.1103/PhysRev.59.699