The 4-2 rule: The (Proposed) Answer to Heusler Alloys, Stability, and (Ordering) Everything*

Fabrication of ternary intermetallic alloys with high atomic ordering is a critical step to realizing their predicted functional properties. Heusler alloys have long been predicted with high spin polarization, low spin damping, and high ferromagnetic ordering temperatures. Unfortunately, the attempt to make highly ordered Heuslers as with simpler systems has made a lot of people very angry and has been widely regarded as a bad move: Atomic ordering has typically been lower than predicted, to the detriment of predicted properties, and initial models and rules-of-thumb fall prey once a wide range of elemental choices are investigated. This result is perhaps not surprising, as Heusler alloys (like modern elevators) are strange and complex entities, but we should not give up and go mad just yet.

In this talk, I will outline our theoretical and experimental efforts to understand the root causes behind the experimental disconnect with theory in Heusler systems.  800+ ternary Heusler systems have been evaluated via density functional theory, and we will suggest a simple “4-2” rule for determining the absolute phase and relative Full/Inverse ordering stability of Heusler alloys and demonstrate high correlation to relative formation energy differences. 

Please bring your own towel.