Dynamical stability of face centered cubic lithium at 25 GPa

Abstract

We study the dynamical stability of face centered cubic lithium at 25 GPa within ab initio density functional theory calculations. The system shows an extremely softened transverse acoustic mode in the Γ–K high symmetry line, whose frequency is difficult to converge within a first-principles harmonic approach. We estimate the anharmonic correction of this value within the frozen phonon approximation assuming that the transverse acoustic mode only interacts with itself. By solving the Schrödinger equation for the potential energy curve and displacements along the vibrational mode of interest, we obtain the anharmonic eigenvalues and eigenfunctions. While the harmonic approach yields a phonon frequency of −28.6 cm⁻¹ for this mode, anharmonicity renormalizes dramatically this value up to 115.3 cm⁻¹. Thus, anharmonic effects seem to dynamically stabilize this system.