Crystallographic disorder refers to a situation in solid materials, particularly in crystalline solids, where there is a deviation from the ideal periodic arrangement of atoms or molecules in the crystal lattice. This disorder can manifest in various forms, such as: 1. **Occupancy Disorder**: Certain atomic sites in the crystal structure may be occupied by different types of atoms or molecules with varying probabilities.
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Crystallographic disorder is variation in atomic positions, orientations or chemical occupation that is represented by an average crystal structure rather than one perfectly repeated local arrangement. A split-site disorder model can describe alternative positions with fractional crystallographic site occupancies. The IUCr disorder-group convention distinguishes atoms present together in one configuration from mutually alternative configurations.[ref]
Atom-site symmetry and disorder in the IUCr core CIF dictionary.
_atom_site_disorder_group, jointly occupied sites and alternative groupsFor an original single-atom model, suppose an atom occupies position0 along an axis with probability0.6 and position with probability0.4. ThenAt , the mean is and the variance is . There need be no atom at the mean position in any individual configuration. Fitting only an average position with a large atomic displacement parameter can conceal this distinction.
The same average occupation could arise from different unit cells adopting different configurations, from motion among configurations during measurement, or from a mixture of these effects. A static average alone does not determine an interchange rate.
Experimental interpretation therefore combines the electron density map, the allowed chemical geometry, occupation constraints and other evidence. Weak or broadened density should not automatically be assigned one specific dynamic mechanism.