Theoretical phase diagram of beryllium at low pressure and high temperature

Abstract

We will try to shed some light on a controversial issue on the phase diagram of beryllium: the very existence of the bcc structure at low pressure and high temperature. In the density functional theory plus quasi-harmonic modeling framework (DFT-QHA), we show that the bcc phase can be dynamically stable at low pressure in a small region delimited by the ρ=2100 kg/m 3 isochore only if large anharmonic effects i) overcome the imaginary N-T 1 mode obtained at T=0 K in the phonon spectrum ii) lead to sufficient amount of entropy to promote the free energy of the bcc phase. Force and velocity averages and melting curves obtained by quantum molecular dynamics (QMD) with 54 atoms in the cell indicate that only the former condition is fulfilled. © 2012 American Institute of Physics.