Diffuse Scattering of X-Rays from Sodium Fluoride

$S\left(\mathrm{class}\right)$ values have been obtained for the monatomic gases argon and neon by Wollan, and for the simple cubic crystals sylvine and rocksalt by Harvey, and Jauncey and May. $F$ values for the average ½(${\mathrm{K}}^{+}$+${\mathrm{Cl}}^{-}$) and ½(${\mathrm{Na}}^{+}$+${\mathrm{Cl}}^{-}$) atoms have been obtained by James and Brindley, and James and Firth. By means of the formulas for gases and crystals developed by Compton, and by Jauncey and Harvey, the above have yielded comparisons between $S\left(\mathrm{class}\right)$ and $f^{\prime }$ values for argon and the average sylvine atoms, and for the average ½(A+Ne) atoms and the average rocksalt atoms. The agreement on the whole has been excellent. The object of the present research was to make similar comparisons of the above results for neon with those for ½(${\mathrm{Na}}^{+}$+${\mathrm{F}}^{-}$), by using Havighurst's $F$ values for this average atom. The methods of experiment and calculation were similar to those of Jauncey and May, as modified by Harvey, except that Woo's form of the crystal formula was used to take account of the incoherent scattering. Unlike the case of argon and sylvine, we found that the $f^{\prime }$ values for the crystal of NaF are definitely lower than those for the gas neon. This implies that for weak nuclear fields the electron distribution in an atom of a crystal is perceptibly more diffuse than that in an atom of the corresponding gas. We also calculated $B$ values and found that the second hump in the $B$ curve which is barely indicated by Wollan's values for neon is definitely present. A Fourier analysis will therefore give a $U$ curve which shows a hump for the $K$ electrons of NaF.