Unusual site preference of Cu in Ni2-based Heusler alloys Ni2CuSb and Ni2CuSn

• Unusual site preference of Cu in Ni2-based Heusler alloys is reported.
• Heusler alloy Ni2CuSb has been synthesized and investigated.
• Valence electrons rule on the site preference in Heusler alloys needs improvement.

The site preference of Cu does not follow the usual valence electrons rule in Ni2-based Heusler alloys Ni2CuSn and Ni2CuSb. The phase stabilities of three possible structures, namely L21, XA and L21B, have been compared. Among them L21 has the lowest total energy and is preferable, while L21B has the highest energy. The energy differences between L21 and the other two structures are quite large. The high phase stability of the L21 structure is mainly related to the strong hybridization between the d states of Cu, Ni and the low N(EF). Single phase, highly-ordered Ni2CuSn and Ni2CuSb samples have been prepared experimentally. The XRD patterns suggest that Ni2CuSn and Ni2CuSb do crystallize in the L21 structure, agreeing with the theoretical prediction. Finally, both of them have a paramagnetic ground state.