Competition of XA and L21B ordering in Heusler alloys Mn2CoZ (ZÂ =Â Al, Ga, Si, Ge and Sb) and its influence on electronic structure

Competition between the highly-ordered XA structure and disordered L21B structure in Heusler alloys Mn2CoZ (Z = Al, Ga, Si, Ge, Sb) has been investigated. The relative stability of the two structures strongly depends on the main group element Z. When Z belongs to Al or Ga, the XA structure is stabler, but when Z belongs to Si, Ge or Sb, the L21B structure gains stability and is lower in energy. This is related to the different number of valence electrons in main group element Z, which influences the DOS structure near the Fermi level and changes N(EF). The energy difference ΔE between the XA and L21B structures may be used to estimate the tendency to form L21B structure in different Heusler alloys qualitatively. A large negative ΔE is preferable to retard the A-C site disorder and retain the highly-ordered XA structure. That is just the case in Mn2CoAl. A robust half-metallicity is observed in Mn2CoSi, Mn2CoGe and Mn2CoSb, they are always half-metallic under either XA or L21B structure. But in Mn2CoAl and Mn2CoGa, their spin gapless semiconducting character will be destroyed and replaced by a half-metallic state if L21B disorder occurs. Finally, these results suggest that the L21B structure should be considered together with XA structure when discussing the electronic structure of “inverse” Heusler alloys.