A new method for phase identification for electron diffractionists.

Abstract

An accurate analytical procedure for phase identification for electron diffractionists has been developed. The method opens new frontiers in the identification of solid-state materials, as crystalline samples in the size range 10 microns to 10 A can be accurately characterized. Research with NIST CRYSTAL DATA (a large database with chemical, physical, and crystallographic data on solid-state materials) has proved that a material can be uniquely characterized on the basis of its lattice and chemical composition. To characterize a material, it is sufficient to determine any primitive cell of the lattice and the element types present. Using a modern analytical electron microscope (AEM), the experimentalist can collect the required data on an unknown sample. The lattice information is obtained by rotation of the sample to obtain two or more planes of data. From these planes, a unit cell defining the lattice can be deduced. The chemical data are determined by energy-dispersive spectroscopy (EDS). Once the experimental data are measured, the unknown is identified against the database of knows using lattice/element-type matching techniques. The basic strategy consists of three conceptual steps. First, the unknown lattice is searched against the database to find all lattices that are the same or related; the results are kept in set 1. Second, the unknown is searched against the database to find all materials with the same or similar element types; the results are kept in set 2. Finally, the results in sets 1 and 2 are combined to obtain the answer set. Experience has proved that the procedure is highly selective and reliable.