  Beryllium (Be) is an important material with wide applications ranging from
aerospace components to X-ray equipments. Yet a precise understanding of its
phase diagram remains elusive. We have investigated the phase stability of Be
using a recently developed hybrid free energy computation method that accounts
for anharmonic effects by invoking phonon quasiparticles. We find that the hcp
to bcc transition occurs near the melting curve at 0<P<11 GPa with a positive
Clapeyron slope of 41 K/GPa. The bcc phase exists in a narrow temperature range
that shrinks with increasing pressure, explaining the difficulty in observing
this phase experimentally. This work also demonstrates the validity of this
theoretical framework based on phonon quasiparticle to study structural
stability and phase transitions in strongly anharmonic materials.
