Bulk aluminum at high pressure: A first-principles study

The behavior of metals at high pressure is of great importance to the fields of shock physics, geophysics, astrophysics, and nuclear materials. We study here bulk crystalline aluminum from first principles at pressures up to $2500\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$---soon within reach of laser-based experimental facilities. Our simulations use density-functional theory and density-functional perturbation theory in the local-density and generalized-gradient approximations. Notably, the two different exchange-correlation functionals predict very similar results for the $\mathrm{fcc}\ensuremath{\rightarrow}\mathrm{hcp}$, $\mathrm{fcc}\ensuremath{\rightarrow}\mathrm{bcc}$, and $\mathrm{hcp}\ensuremath{\rightarrow}\mathrm{bcc}$ transition pressures, around 175, 275, and $380\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, respectively. In addition, our results indicate that core overlaps become noticeable only beyond pressures of $1200\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. From the phonon dispersions of the fcc phase at increasing pressure, we predict a softening of the lowest transverse acoustic vibrational mode along the [110] direction, which corresponds to a Born instability of the fcc phase around $725\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$.