Optical properties and electronic structures ofB2andB19′phases of equiatomic Ni-Ti alloys

Abstract

The dielectric functions of equiatomic Ni-Ti alloys were measured by spectroscopic ellipsometry in the energy range of 1.5--5.4 eV at $\ensuremath{\sim}423$ and at $\ensuremath{\sim}25 \mathrm{K}.$ The peak at $\ensuremath{\sim}2.26 \mathrm{eV}$ in the ${B19}^{\ensuremath{'}}$ (monoclinic structure) optical conductivity spectrum has a slightly larger magnitude than in the $B2$ (cubic CsCl structure) phase, while the shoulder at $\ensuremath{\sim}3.5 \mathrm{eV}$ becomes weaker and almost indiscernible upon martensitic transformation. A new structure develops at $\ensuremath{\sim}2.85 \mathrm{eV}$ in the ${B19}^{\ensuremath{'}}$ phase; however, it is also very weak. The band structures and the optical conductivity were calculated in both phases using the linearized-augmented-plane-wave method within the local-density approximation. $\mathbf{k}$ points near the $\ensuremath{\Gamma}\ensuremath{-}X\ensuremath{-}M$ plane in the $B2$ phase and the corresponding $\mathbf{k}$-points in ${B19}^{\ensuremath{'}}$ phase contribute significantly to all three structures. The difference between the two spectra is due to the transitions between the folded-back bands from the $B2$ phase because of the larger unit cell of the ${B19}^{\ensuremath{'}}$ phase and the change in the electronic energy spectrum near the Fermi level. The overall optical properties of Ni-Ti alloys in the measured energy range are rather insensitive to the martensitic transformation because the states far from the Fermi level are mainly involved in the interband transitions.