Optical properties and electronic structures of B2 and B19 ' phases of equiatomic Ni-Ti alloys

The dielectric functions of equiatomic Ni-Ti alloys were measured by spectr oscopic ellipsometry in the energy range of 1.5-5.4 eV at similar to 423 an d at similar to 25 K. The peak at similar to 2.26 eV in the B19' (monoclini c structure) optical conductivity spectrum has a slightly larger magnitude than in the B2 (cubic CsCl structure) phase, while the shoulder at similar to 3.5 eV becomes weaker and almost indiscernible upon martensitic transfor mation. A new structure develops at similar to 2.85 eV in the B19' phase; h owever, it is also very weak. The band structures and the optical conductiv ity were calculated in both phases using the linearized-augmented-planewave method within the local-density approximation, k points near the Gamma-X-M plane in the B2 phase and the corresponding k-points in B19' phase contrib ute significantly to all three structures. The difference between the two s pectra is due to the transitions between the folded-back bands from the B2 phase because of the larger unit cell of the B19' phase and the change in t he electronic energy spectrum near the Fermi level. The overall optical pro perties of Ni-Ti alloys in the measured energy range are rather insensitive to the martensitic transformation because the states far from the Fermi le vel are mainly involved in the interband transitions. [S0163-1829(99)06403- 6].