Temperature- and pressure-induced crystal phase transitions in Be

Abstract

Beryllium is known to undergo a HCP-BCC transition at about 1530K and ambient pressure. This transition is studied theoretically by comparing the Gibbs free energies for the HCP, FCC and BCC modifications of Be. The static lattice energy is calculated using the ab initio pseudopotential method. The photon energy and entropy are estimated from the calculated elastic constants. A mechanism for this transition is proposed based on the results of the calculations. At high temperature, the BCC modification is found to be stabilised by the large entropy for this phase caused by the presence of soft phonon modes. Possible pressure-induced phase transitions are examined. At about 1-2 Mbar the FCC and BCC phases are both more stable than the HCP phase. The regions of FCC and BCC stability lie too close to be separated in the present calculation.