Lattice instabilities in hexagonal NiSi: A NiAs prototype structure

Abstract

We report a first-principles study of the hexagonal NiSi phase with the B81 strukturbericht designation. This structure, reported by Foll Philos. Mag. A 45, 31 1982, d’Heurle J. Appl. Phys. 55, 4208 1984, and Dai Appl. Phys. Lett. 75, 2214 1999, is actually only observed during annealing of Ni films on 111 silicon crystals. We discuss, in this paper, about its structural, energetic, vibrational, electronic, and elastic properties, computed by means of the density-functional and density-functional perturbative theory within the spinpolarized Perdew-Burke-Ernzerhof functional. Two configurations with this crystallographic structure have been studied, noted h-NiSi and h-SiNi in the following. We show that theoretical and experimental lattice parameters are not compatible for both systems. A large discrepancy 8–10 % is evidenced, much larger than both experimental and simulation accuracies obtained for others Ni-Si systems. Moreover the vibrational spectra of h-NiSi and h-SiNi present both soft modes, indicating that in their ground states these systems are dynamically unstable. Using a band folding approach, we have analyzed modes for h-NiSi on a supercell, permitting us to identify eigenvectors associated to these instabilities. We have then relaxed the cell in accordance to these eigenvectors, and a final structure is thus proposed. To understand the mechanism at the origin of these negative frequencies in h-NiSi, electronic states around the Fermi level have been plotted, and we identify in the Fermi-surface potential nesting vectors, suggesting that an electron-phonon coupling mechanism could be at the origin of the instability. Whereas the ground state of “h-NiSi” seems not to be associated to the B81 system, we show that a stress in the basal plane could induce an increasein the c axis, restoring the agreement with experimental data.