Figure 2.

The relationship between mosaicity and mosaic block size. Ewald sphere diagram identifying the reciprocal-lattice points (gold) actually observed in the diffraction pattern. A goal of modeling is to adjust the effective mosaicity (a) and effective mosaic domain size (b) together so as to bring all the observed points into contact with the sphere of reflection of radius 1/λ (where λ is the wavelength), but not the unobserved (blue) points. (a) Mutual rotation of mosaic blocks spreads the points into concentric arcs (spherical caps in three dimensions) subtending a constant angle η at the reciprocal-lattice origin O, with η interpreted as the full-width effective mosaicity. A lattice point diffracts if its centroid (midpoint) can be brought onto the sphere of reflection with a rotation Δψ ≤ η/2. (b) Expansion of the reciprocal-lattice points into constant-sized spheres, reflecting the finite size of mosaic blocks (Nave, 1998 ▶) or, equivalently, the domain-size broadening (Scherrer, 1918 ▶). The sphere diameter α is inversely proportional to the effective mosaic block size D _eff. The sphere size illustrated in (b) falls short of that needed to completely model the observed reflections.