Elastic and phonon instabilities in simple-cubic heavier alkali metals K, Rb and Cs under hydrostatic pressure studied using ab initio calculations

Abstract

Using first-principles calculation based on density functional theory, the elastic stability and dynamic stability of heavier alkali metals K , Rb and Cs in sc structure are investigated. Results reveal that the C44 instabilities are responsible for the mechanical instabilities for these heavier alkali metals and the spin-polarization phases are more stable than non-spin-polarization phases. Other than K - sc which presents BZ edge instability at the M-point at ambient condition, the K - sc and Rb - sc are dynamically stable for the range of the pressure in our calculation. But for Cs - sc , it becomes unstable starting from 7 GPa. Especially we demonstrate that the most stable phases of K at low temperatures and pressures around 20 GPa are ferromagnetic K in sc structure.