Measurement of Body-Centered-Cubic Aluminum at 475 GPa

Abstract

Nanosecond in situ x-ray diffraction and simultaneous velocimetry measurements were used to determine the crystal structure and pressure, respectively, of ramp-compressed aluminum at stress states between 111 and 475 GPa. The solid-solid Al phase transformations, fcc-hcp and hcp-bcc, are observed at 216±9 and 321±12 GPa, respectively, with the bcc phase persisting to 475 GPa. The high-pressure crystallographic texture of the hcp and bcc phases suggests close-packed or nearly close-packed lattice planes remain parallel through both transformations.

Toroidal diamond anvil cell for detailed measurements under extreme static pressures

“…The transition to a bcc phase has been reported very recently around 321 GPa in a laser ramp-compression experiment 26 . The present data on Al thus offers the possibility of direct comparison between static and dynamic measurements of the phases and EoS of a metal under extreme compression.…”

“…The dashed lines are the equations of state of the fcc and hcp phases measured under non-hydrostatic compression in a standard diamond anvil cell 27 , after correction of the pressure metrology (see text). The crosses are volume vs. stress points measured in recent ramp-compression experiments 26 , and the gray line the ab initio predicted 0 K equation of state 47 . c c / a ratio in hcp-Al measured here and by ramp compression 26 …”

“…The crosses are volume vs. stress points measured in recent ramp-compression experiments 26 , and the gray line the ab initio predicted 0 K equation of state 47 . c c / a ratio in hcp-Al measured here and by ramp compression 26 …”

“…The bcc phase appears at a slightly lower pressure with dynamic compression than in t-DAC (321 vs. 360 GPa), which can be due to a temperature effect. Indeed, the pressure–temperature path followed during ramp compression is not well controlled, but is always higher in temperature than the principal isentrope 48 , which is estimated to get through 920 K at 364 GPa 26 . Temperature increase usually favors the bcc phase over close-packed phases through the entropy term 49 , resulting in negative Clapeyron slopes for hcp → bcc transformations 47 .…”

“…The capability to perform in situ X-ray diffraction on ramp-compressed solids was then developed, and compression curve and structural changes in solids up to the TPa range are now measured 25 . That has recently been illustrated by the observation of the face-centered cubic–hexagonal close-packed–body-centered cubic (fcc–hcp–bcc) phase transitions in aluminum (Al) 26 , which had remained a long-pursued project for the static approach 27 .…”