The Scattering of X-rays by Gases and Crystals

Recently A. H. Compton and Jauncey and Harvey have obtained theoretical formulas for the scattering of x-rays by gases and crystals respectively. By means of these formulas f^' values can be calculated. From measurements on the diffuse scattering by rocksalt, Jauncey and Harvey have calculated f^' values for the average atom, or (Na⁺+Cl^-)/2, in rocksalt. Wollan has also recently obtained f^' values for neon and argon. The f^' values for the average of neon and argon, or (Ne+A)/2 are compared with those for (Na⁺+Cl^-)/2 and an excellent agreement is found. Also these two sets of experimental f^' values are shown to be in excellent agreement with the quantum mechanics model of Waller and James. It is shown that the difference between f^' and f, the true atomic structure factor, is not negligible and that the error in assuming f to be given by f^' may be as large as 20 or 30 percent at large values of (sinφ2)λ. Also it is pointed out that only f, but not f^', values may be used in the Fourier analysis method of unravelling crystal structure.