A new approach to alchemi: the ordering tie line method

Abstract

The ALCHEMI (Atom Location by CHanneling Enhanced Microanalysis) technique has been first developed by Spence and Taftø to determine site occupancies of alloying elements in ordered crystals. Based on their work, a number of formulations have been developed to improve'the accuracy by using multivariant analysis, to extend the analysis to a more general case, and to incorporated anti-site defects and derealization effect. However, all these formulations can be replaced by a new formulation based on the Ordering Tie Line (OTL) concept.The OTL concept can be illustrated by using a ternary compound (C°=AxByCz), with a B2 crystal structure, as an example. In the B2 crystal structure (Fig.1), the sublattices of α and β can be treated as two ' phases" with compositions Cα and Cβ (for B2 ordering ⇔ Cα ≠ Cβ). The OTL is then defined as the tie-line connecting Cα and Cβ(Fig. 1). Similar to a regular tie-line, the OTL describes mass conservation in a B2 crystal and its mid point will locate at C°.