Electronic structure and structural transformation of Co1-xNi1+xAl (x=0, ∓1/2)

To expand our knowledge of the martensitic transformation from a B2 to an Ll(0) structure for ferromagnetic shape memory Co-Ni-Al alloys, we have performed first-principles band calculations for the supercell structures corresponding to Co1-xNil+Al-x (x =0, +/- 1/2) in a nonmagnetic state. From the investigation of total energy vs c/a, we have found that the total-energy gain for x = 0 due to the distortion is 3 similar to 4 times as large as those for x = +/- 1/2. This result indicates that the stabilization of an Ll(0) structure becomes weak in a rich Co or Ni region compared with the composition of Co:Ni:Al = I:I:I. We have discussed the result in terms of band energy and such components as CoAl, NiAl, CoNi and etc., which compose Co1-xNi1+xAl.