Origin of diffuse scattering appearing in iron-doped Ti-Ni shape memory alloys

We have investigated diffuse scattering appearing in Ti-Ni based shape memory alloys to understand the origin of instability of the B2-type structure in the system. Transmission electron microscope observation and a first-principle electronic structure calculation have been made for Ti-(50-x)Ni-xFe (x = 6, 7, 8, 10). They exhibit diffuse scattering in the electron diffraction pattern along 〈ξξ0〉* direction on cooling with the intensity peak located at incommensurate positions of q_diffuse = 〈ζζ0〉*. The value ζ increases with decreasing temperature for all alloys. The temperature at which the incommensurate scattering start to appear is close to the one at which the electrical resistivity exhibits a local minimum, T_min, for all the alloys. The value of q_diffuse at T_min is nearly the same as the nesting vector of Fermi surface at T_min, which is obtained by calculating the generalized susceptibility of the B2-type structure along the 〈ξξ0〉* direction. These results strongly suggest that the nesting effect of Fermi surface is responsible for the instability of the B2-type structure in Ti-Ni based shape memory alloys.