W0143

Democracy in the Imperfect World: Local Crystallography of Crystals with Disorder. T. Egami, Univ. of Pennsylvania, Philadelphia, PA.

The lattice periodicity of a crystal produces a number of wonderful, magical effects, and forms the basis for crystallography. However, real crystals usually are not perfectly periodic, and include various deviations from periodicity. In simple metals these deviations are either lattice defects, of which density is rather low, or segregation of a second phase. In such a case the conventional methods of crystallographic analysis can give adequate descriptions in terms of diffuse scattering to describe strains and multi-phase fit for crystal structure. However, the materials of technological or scientific importance today, such as complex oxides and organic crystals, have much more extensive atomic scale disorder that cannot be properly described by the crystallographic methods. An alternative approach is to take a local view, by placing the reference point on the atom, just as the democracy places the reference point on each people. Certain aspects of the real society is understood only from a personal, local point of view, rather than by the bird’s view from above. The method of atomic pair-density function (PDF) analysis is one of such approaches, and in recent times has been successfully applied to a number of interesting systems, some of which are covered in this symposium. B. E. Warren was one of the early, strong proponents of this method. It is quite befitting to have the PDF approach a central theme of this symposium to celebrate his foresights.