Diffuse Scattering of X-Rays from Sodium Fluoride. II. Scattering at the Temperature of Liquid Air

The ratio of the intensity of x-rays diffusely scattered by a single crystal of NaF at 86°K to that of x-rays scattered by the crystal at 295°K has been determined at each of several angles ranging from 10° to 42°. The x-rays consisted of a continuous band of wave-lengths, of which the spectral distribution of intensity was known. From Shonka's value of 442°K for the characteristic temperature of NaF together with his $f$ values for zero point energy, we have calculated the theoretical ratios for different angles. The experimental and theoretical ratios agree very well. The importance of being sure of the absence of Laue spots in this type of experiment is discussed. The experiment was performed by fixing the scattering angle $\phi$ and varying the crystal angle $\theta$ as in the original method of Jauncey and May. If $\theta$ is kept constant and $\phi$ varied there is no sure criterion for the absence of Laue spots. A formula for the ratio at zero scattering angle is given and at small angles the experimental ratio should approach this value. Curves showing the contrast in behavior of the Laue, diffuse and "total" (see Jauncey and Pennell) scattering from NaF and KCl with change of temperature from 295°K to 86°K are given.