Crystal structure and site preference, magnetic and elastic properties, and martensitic transformation of Ni- and Fe-doped Co2VGa alloys: A first-principles study

Abstract

Using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, the crystal structure and site preference, magnetic and elastic properties, and martensitic transformation (MT) are systematically investigated with the three groups of Heusler alloys: (Co2−xMx)VGa (M1x), Co2(V1−xMx)Ga (M2x), and Co2V(Ga1−xMx) (M3x, M = Ni and Fe, 0≤x≤1.0). It is shown that instead of the L21 and XA structures, the fcc one is energetically preferred in the cubic M3x (x≥0.8) alloys. In L21-Ni2x (x≤0.6) and fcc-Ni3x (x=0.8), Ni atoms even prefer the Ga and Co anti-sites, respectively, and the replaced atoms move to the sublattices of the deficient ones. Their total magnetic moment is dominated by the magnetic exchange interactions corresponding to the pairs of two Co atoms on the different sublattices in M = Ni and Fe1x, Co and Fe in Fe2x and Fe3x (x<0.8), and Fe and Fe atoms in Fe3x (x≥0.8) alloys, respectively. These Ni1x, Ni2x, and Fe3x with x≥0.4 as well as Ni3x with x≥0.2 alloys are predicted having the MT behavior and also the better mechanical property relative to Co2VGa. A lower shear modulus (C′=(C11−C12)/2) generally corresponds to a higher MT temperature, and these alloys, which can undergo the MT are further evaluated with C′<36.50 GPa. Both considerable magnetocaloric and magnetovolume effects can be also expected during the MT of these Fe3x alloys (x=0.4 and 0.6). In the remaining Fe1x and Fe2x alloys, the Fe doping disfavors the MT and also improves their brittleness. The structural preference of these cubic alloys and also their stability relative to the tetragonal martensite can be mainly attributed to the number of their minority density of states at the Fermi level: the smaller they are, the more stable their system tends to be.