Structural phase stability and electron-phonon coupling in lithium

First-principles calculations of the free energy of several structural phases of Li are presented. The density-functional linear-response approach is used to calculate the volume-dependent phonon frequencies needed for computing the vibrational free energy within the quasiharmonic approximation. We show that the transformation from a close-packed structure at low temperatures to the bcc phase upon heating is driven by the large vibrational entropy associated with low-energy phonon modes in bcc Li. In addition, we find that the strength of the electron-phonon interaction in Li is strongly dependent on crystal structure. The coupling strength is significantly reduced in the low-temperature close-packed phases as compared to the bcc phase, and is consistent with the observed lack of a superconducting transition in Li.