Abstract
Previous workers have reported that lattice constants determined by electron diffraction differ from the accepted X-ray values. It has been suggested that these differences may be due to a change in lattice constant with crystal size. To examine this question further, the lattice constants of several alkali halides have been determined, by electron diffraction, to a higher accuracy than had previously been attained. The X-ray value of the lattice constant of TlCl was used as a standard reference. The lattice constants of six alkali halides, with crystal sizes greater than 150 A, were found to agree with the X-ray values to within the experimental error of 0.05% It has been shown that for crystal sizes less than 100 A the lattice constants of the alkali halides were found to be less than the X-ray values by an amount dependent on crystal size For a crystal size of about 30 A the decrease was approximately 0.5%
Similar determinations on evaporated gold films showed that, for a crystal size of about 20 A., the decrease in the lattice constant was almost 2% For a crystal size of 40 A., the largest obtained, the lattice constant was 0.2% less than the X-ray value. Evaporated films of silver and bismuth also were found to have lattice constants less than the X-ray values.
It can therefore be concluded that previous reports of an increase of the lattice constants of alkali halides with decreasing crystal sizes are in error Some of the discrepancies noted by previous workers may have resulted from assigning the X-ray value to the lattice spacing of gold films or gold foil used as a standard reference.