Spontaneous symmetry breaking of substitutional solute atoms in dilute a-Ti solid solutions

According to classical metal physics theory, the substitutional solute atom in dilute metal solid solution occupies the high-symmetry lattice site without destroying the point group symmetry of the host lattice. However, our recent first-principles calculations showed that the substitutional solute atom Mo occupies the low-symmetry off-center position in dilute Ti solid solutions such that breaking spontaneously the point group symmetry of the hexagonal-close-packed host lattice [Q. M. Hu and R. Yang, Acta Mater. 197, 91 (2020)] resulted from the Jahn-Teller splitting of the degenerated d orbitals of Mo at the Fermi level. Whether the spontaneous symmetry breaking is general for some other solid solutions and robust (e.g., to the temperature effect) remains an open question. In the present work, we investigated the site occupation of substitutional atom X in dilute Ti-X solid solution with X being all the 3d metal elements from V to Zn. Among the considered solute atoms, Cr, Mn, Fe, and Co are dynamically and energetically stable to occupy the low-symmetry off-center site such that the spontaneous symmetry breaking occurs, whereas V, Ni, Cu, and Zn prefer to stay at the high-symmetry lattice site. We demonstrated that the off-center occupation is robust against the supercell size adopted in the first-principles calculation and the thermal volume expansion.

Automated summary

According to classical metal physics theory, the substitutional solute atom in dilute metal solid solution occupies the high-symmetry lattice site without destroying the point group symmetry of the host lattice. However, recent first-principles calculations revealed that the substitutional solute atom Mo occupies a low-symmetry off-center position in dilute Ti solid solutions, which breaks the point group symmetry of the hexagonal-close-packed host lattice due to the Jahn-Teller splitting of the degenerated d orbitals of Mo. In this study, we investigated the site occupation of substitutional atom X in dilute Ti-X solid solution with X ranging from V to Zn, and found that Cr, Mn, Fe, and Co exhibit spontaneous symmetry breaking by occupying the low-symmetry off-center site, while V, Ni, Cu, and Zn tend to stay at the high-symmetry lattice site, and this off-center occupation is robust against the size of the supercell and the thermal volume expansion in first-principles calculations.