The dielectric functions of equiatomic Ni-Ti alloys were measured by spectroscopic ellipsometry in the energy range of 1.5–5.4 eV at $\sim 423$ and at $\sim 25\hspace{0.5em}\mathrm{K}.$ The peak at $\sim 2.26\hspace{0.5em}\mathrm{eV}$ in the ${B19\mathrm{}}^{\prime }$ (monoclinic structure) optical conductivity spectrum has a slightly larger magnitude than in the $B2\mathrm{}$ (cubic CsCl structure) phase, while the shoulder at $\sim 3.5\hspace{0.5em}\mathrm{eV}$ becomes weaker and almost indiscernible upon martensitic transformation. A new structure develops at $\sim 2.85\hspace{0.5em}\mathrm{eV}$ in the ${B19\mathrm{}}^{\prime }$ 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 $\Gamma -X-M$ plane in the $B2\mathrm{}$ phase and the corresponding $\mathbf{k}$-points in ${B19\mathrm{}}^{\prime }$ 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 $B2\mathrm{}$ phase because of the larger unit cell of the ${B19\mathrm{}}^{\prime }$ 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.

• Received 13 July 1998

DOI:https://doi.org/10.1103/PhysRevB.59.1878