Electronic structure and stability of Ti-based B2 shape-memory compounds: X-ray and ultraviolet photoelectron spectra.

Abstract

The evolution of x-ray photoelectron and ultraviolet spectra and their modification at the phase transformation B2-B19(B19') of equiatomic TiM and quasibinary Ti(Ni,M) compounds, where M stands for Fe, Co, Ni, Pd, Pt, Au, and Cu, are presented. Investigation of x-ray photoemission spectroscopy includes valence bands, satellites, and core-level energy studies. As the atomic number of M increases (both within the same period and along the group), its d states become more localized, the maximum and the center of gravity of the d bands shift towards the bottom of the valence band, and the M contribution to the density of states (DOS) at the Fermi level, N(${\mathit{E}}_{\mathit{F}}$), degrades. The DOS localization is accompanied by a spatial localization of the M d electrons resulting in weakening of the d-d covalent bonds between the alloy components and, thus, destabilizing the B2 phase. While the contribution of M d electrons to the DOS at ${\mathit{E}}_{\mathit{F}}$ decreases, the Ti d electron portion increases to such an extent that, e.g., in TiPd and TiAu, the M contribution to N(${\mathit{E}}_{\mathit{F}}$) is almost negligible. The increase in core-level energy of all partners in compounds with respect to the elemental state is discussed in terms of covalency. The M d band localization leads to an increase of many-body interactions and, as a consequence, we managed to observe the Pd satellites without resonance enhancement. The B2-B19' phase transition is accompanied by modification of the DOS which affects the Ti d states in general. Having analyzed the electronic structure we introduce parameters controlling the B2 phase stability and suggest a phenomenological formula for the martensitic-transformation temperature which satisfactorily describes the experimental behavior.