EFFECTS OF SURFACE RELAXATION ON POWDER DIFFRACTION PATTERNS OF VERY FINE PARTICLES

Calculations of powder diffraction peaks using a model of tiny (diameter a few nm) spherical crystals log normally distributed in size with and without surface relaxation are presented. The calculated peak shapes are more nearly Gaussian than Cauchy (Lorentzian). Surface relaxation can produce small variations in peak positions, order-dependent peak widths and asymmetric tails. The latter may be quite in the smaller particles and are similar to recent data taken on nanometre-sized crystals. This suggests that comparison of model calculations with X-ray powder data of single peaks or, preferably, entire diffraction patterns (perhaps as part of a Rietveld refinement) may allow probing of surface atom positions in nanometre-sized particles.