A Study of X-Ray Line Broadening Due to Strain and Particle Size in Some Alkali Halides.

A number of alkali halides and similar structures have been studied which show x-ray line broadening characteristics similar to those which have been reported for metals. Broadening was mainly obtained by ball milling and could be reduced to a minimum instrumental effect by annealing. The broadening increased with the order of the reflection but it showed a deviation from a smooth curve which was characteristic of the substance and was related to the elastic constants. Analysis of the cause of broadening showed it usually to be due to strain alone, but in the case of lithium fluoride, particle size as well as strain contributed appreciably to the broadening. Mean values of strain along the principle crystallographic axes were determined, and from computed values of Young's modulus a corresponding mean stress function was obtained. The degree of induced strain was found to be inversely related to the unit cell size and the melting point. Strain was also found to affect the radiation colouring. Lithium fluoride also showed a change in parameter when annealed, and this change remained constant when the annealing temperature exceeded 300° C. Calcium fluoride, which like lithium fluoride, showed considerable particle size broadening as received from the manufacturer, also showed a parameter change with annealing temperature, with a similar discontinuity just below 300° C. It is suggested that the strain release up to 300° C in the two substances is associated with a segregation of impurities, whilst above 300° C the strain results from a relaxation of lattice distortion. Two methods of profile analysis are in common use and the relative merits of these are discussed. By cold working annealed calcium fluoride it was found possible to produce a specimen showing profile integral breadths equal to the original material, but which analysis showed to be caused almost entirely by strain