Occupation preferences and impacts of interstitial H, C, N, and O on magnetism and phase stability of Ni2MnGa magnetic shape memory alloys by first-principles calculations

Microalloying by introducing small atoms into the interstitial sites of crystal represents an important strategy in composition design, usually enabling a leap in material performance under a tiny doping concentration. However, for the Ni–Mn-based magnetic shape memory alloys, plenty of critical scientific issues related to interstitial alloying remains ambiguous. In this work, by first-principles calculations, the occupation preferences, and the impacts and the underlying mechanisms of H, C, N, and O on magnetism, phase stability, and electronic structures of Ni2MnGa, were systemically investigated. By using a two-stage relaxation strategy, it is confirmed that all the studied interstitial atoms prefer to occupy the octahedral interstice, although the undistorted octahedral interstice possesses a smaller size than that of the tetragonal interstice. The magnetic moments of Ni and Mn around the interstitial element are highly modified, which is attributed to the decreased concentration of conduction electrons, resultant from the formation of covalent bonds between Ni and the interstitial atoms, and the revised distances between Mn–Ni(Mn) caused by the local lattice distortion. Interstitial alloying can highly tailor the phase stability and the c/a ratio of martensite. The doping of C has a great potential to destabilize the austenite owing to the reduced ferromagnetism, which is opposite to the case in steel. The atom radius of an interstitial atom may be the critical factor dominating the elastic stability of the alloyed systems. This work is expected to provide fundamental information for interstitial alloying to promote the design of advanced magnetic shape memory alloys.