Competition of L21 and XA structural ordering in Heusler alloys X2CuAl (XÂ =Â Sc, Ti, V, Cr, Mn, Fe, Co, Ni)

A competition between the conventional L21 ordering and inverse XA ordering has been observed in Heusler alloys X2CuAl (X = Sc, Ti, V, Cr, Mn, Fe, Co, Ni). In X2CuAl, the site preference of Cu is strongly influenced by the crystal structure of pure X elements. When X element has a close-packed FCC or HCP structure (Sc, Ti, Co and Ni), Cu prefers entering the (14,14,14) position and forms the L21 structure, but when X has a BCC structure (V, Cr, Mn, Fe), Cu prefers entering the (12,12,12) position and forms the XA structure. So the crystal structure of pure X element should also be considered together with number of valence electrons when discussing the site preference in Heusler alloys. Mn2CuAl is found to be a spin gapless semiconductor (SGS) with fully-compensated total moment. Based on this, a possible rule to design SGS has been discussed. Finally, X2CuAl alloys all have a negative formation energy except for V2CuAl and Cr2CuAl.