Bonding origin of abnormal lattice-volume dependence of phase stability in magnetic shape memory alloys

Lattice volume dependence of phase stability, i.e. , the contracted lattice of crystal tends to elevate the temperature of phase transition, is a well-known criterion to guide materials design in the Ni–Mn(Fe)-based magnetic shape memory alloys. However, there exist some confusing exceptions, such as Ni 2 Mn(Ga, Al), which are not well understood. In this work, the structural stabilities, phonon curves and electronic structures of the isoelectronically substituted Ni 2 MnGa 1-x Al x (x=0∼1) alloys were studied by first-principles calculations. We find that the abnormal structural stability in the examined alloys is an intrinsic effect resulting from the variation of chemical composition. Al element has an intrinsically stronger ability to stabilize the austenite compared with Ga. In terms of electronic structures, both the Fermi surface nesting and the minority-spin electronic states near the Fermi level, the usual driving forces of structural transition, are insensitive to Al substitution. Bonding analyses indicate that the reduced covalent hybridization between Ni and sp elements (Al, Ga) governs the structural transformation. The ionic interactions between Ni and sp element also affect the phase stability. These covalent and ionic interactions could be the origin of the abnormal dependence of structural stability on lattice volume in the studied alloys. The results of this work clarify the abnormal lattice-volume-related phase stability and thus lay a theoretical foundation for the design of advanced magnetic shape memory alloys.