Bulk aluminum at high pressure: A first-principles study

Abstract

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 firstprinciples at pressures up to 2500 GPa -soon within reach of laser-based experimental facilities. Our simulations use density-functional theory and density-functional perturbation theory in the localdensity and generalized-gradient approximations. Notably, the two different exchange-correlation functionals predict very similar results for the f cc → hcp, f cc → bcc, and hcp → bcc transition pressures, around 175 GPa, 275 GPa, and 380 GPa respectively. In addition, our results indicate that core overlaps become noticeable only beyond pressures of 1200 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 GPa.First-principles calculations have proved useful to the fields of geophysics, 1 astrophysics, 2 and nuclear materials. 3 Aluminum, being cubic close-packed and having no d-shell electrons, is a prototype for theoretical predictions and understanding the high-pressure behavior of simple metals. 4 Currently the National Ignition Facility 5 at LLNL is expected to achieve shockless compression 6 of metals up to 2000 GPa. This new facility may provide rapid advancements to high-pressure physics and could partner very successfully with theoretical studies.The equation of state (EOS) and phase stability of aluminum were first studied from first-principles in the early 1980s. 7,8,9 In all cases the predicted phase sequence was f cc → hcp → bcc, but predictions differed greatly in the transition pressures. Several other calculations within the local-density approximation (LDA) 10 or the generalized-gradient approximation (GGA) 11,12 have since then been performed, with a predicted static (i.e. without the phonon contribution) f cc → hcp transition pressure of 205 ± 20 GPa 10 in LDA and 170 GPa 11 and 192 GPa 12 in GGA. These discrepancies are more notable for the hcp → bcc transition pressure: 565 ± 60 GPa 10 in LDA versus 360 GPa 11 in GGA, leaving significant uncertainties open. Theoretical work on the vibrational properties of aluminum also suggests for the f cc → hcp transition a transition pressure higher than the static one. 11,12 Elastic properties 13,14 and the absolute strength under tension 15 have also been calculated; the latter results are of particular interest as they demonstrate the important role vibrational modes play in determining mechanical stability and suggest that shear failure modes are inherent in aluminum.Experimentally, the equation of state of aluminum at high pressures was studied by shock-compression 16 at pressures above the predicted maximum for the f cc → hcp phase boundary, 10 but a transition was not observed. However recent diamond anvil cell experiments observed a f cc → hcp transition at 217 ± 10 GPa 17 ...

Multiphase aluminum equations of state via density functional theory

“…These results are consistent with earlier DFT calculations 17,18 . Table I lists the calculated parameters of the cold curves, i.e., the equilibrium density ρ, the bulk modulus, B, and its pressure derivative B ′ , along with the Debye temperature θ D (evaluated from the phonon spectra described below).…”

“…For the temperatures and densities ranges we calculated, it was sufficient to perform gamma point calculations. For densities above 8.5 g/cm 3 and temperatures at 6 eV and above we used an 11-electron norm conserving pseudopotential 17 , which enabled extension to densities of 13.5 g/cm 3 . The plane wave energy cutoffs were 30 Ry (1 Ry = 13.605 eV) for the 3-electron PAW and 100 Ry for the 11-electron norm conserving pseudopotential.…”

Thermal conductance of metal–diamond interfaces at high pressure

“…. This extrapolation is in fair agreement with prior theoretical results for Al, Pt and Au (points and dot-dash lines) [35][36][37] . At 50 GPa, our Debye extrapolation deviates from the theoretical o c of Al, Pt and Au by þ 11%, À 6% and À 8%, respectively.…”

“…Figure 3 shows the resulting o c and compares them to more sophisticated theoretical and first-principles calculations for Al 35 , Pt 36 and Au 37 . At 50 GPa, our Debye extrapolation deviates from the theoretical o c of Al, Pt and Au þ 11%, À 6% and À 8%, respectively.…”

Evidence of superdense aluminium synthesized by ultrafast microexplosion

“…We calculate atomic radii of Al and O at two pressures: 100 GPa—the estimated maximum remnant pressure of bcc-Al in Al2O3 matrix after the laser pulse and 275 GPa—the theoretically predicted pressure for bcc-Al formation41. Following the EoS obtained by Akahama et al 22,.…”