L21 and XA ordering competition in titanium-based full-Heusler alloys

The site preference rule, i.e., that the atomic sites of transition-metal-elements X and Y are determined by the number of their valence electrons, has been widely used in the design of full-Heusler alloys X(2)YZ and also used to explain their properties. In this work, the most popular Ti-2-based Heusler alloys are selected as targets to study the site preferences of their atoms by theoretical calculations. It is observed that most of them are likely to form the L2(1)-type structure instead of the XA one. The reason for the site preference is explained on the basis of the calculated charge density differences. We further prove that each alloy shows abruptly different spintronic properties, depending on its L2(1)-type or XA-type structures. This research can be regarded as a counterexample to the site preference rule and is instructive for the future design of full-Heusler alloy materials.