ALCHEMI: a new technique for locating atoms in small crystals

TLDR

ALCHEMI is a quantitative technique for locating crystallographic sites, distribution, and types of substitutional impurities in crystals. The method determines atom positions by measuring the orientation dependence of characteristic X‑ray emission with an energy‑dispersive microanalyser on a TEM, without requiring specimen thickness or precise orientation and with minimal assumptions about the dynamical electron wavefunction. The technique, which needs no adjustable parameters, can analyze regions as small as a few hundred Å, detect impurity concentrations as low as ~0.1 at %, distinguish neighboring elements in the periodic table, and has been applied to study cation ordering in spinels, feldspars, and olivine.

Abstract

SUMMARY Atom Location by Channelling Enhanced Microanalysis (ALCHEMI) is a quantitative technique for identifying the crystallographic sites, distribution and types of substitutional impurities in many crystals. The method involves no adjustable parameters, can be applied to areas as small as a few hundred Angstroms and to impurity concentrations down to about 0·1 atomic per cent. It is capable of distinguishing neighbours in the periodic table. The method uses the incident electron beam orientation dependence of characteristic X‐ray emission and uses an energy dispersive X‐ray microanalyser fitted to a transmission electron microscope. The method does not require the specimen thickness or precise orientation to be known, and makes few assumptions about the form of the dynamical electron wavefunction, which need not be calculated or predicted. The classical problems of cation ordering in spinels, feldspars and olivine have now been studied by this method.