What is "mosaicity"

n. 1 (context physics chemistry English) A measure of the long-range disorder of a crystal 2 (context medicine English) The presence of mosaic vessels in a tumour

In crystallography, mosaicity is a measure of the spread of crystal plane orientations. A mosaic crystal is an idealized model of an imperfect crystal, imagined to consist of numerous small perfect crystals ( crystallites) that are to some extent randomly misoriented. Empirically, mosaicities can be determined by measuring rocking curves. Diffraction by mosaics is described by the Darwin-Hamilton equations.

The mosaic crystal model goes back to a theoretical analysis of X-ray diffraction by C. G. Darwin (1922). Currently, most studies follow Darwin in assuming a Gaussian distribution of crystallite orientations centered on some reference orientation. The mosaicity is commonly equated with the standard deviation of this distribution.