Dense close-packed phase of tin above 157 GPa observed experimentally via angle-dispersive x-ray diffraction

A hexagonal close-packed ($P63/mmc$) phase of the group 14 element Sn has been observed using angle-dispersive x-ray diffraction. The phase transition proceeds from bcc→hcp above 157 GPa at 298 K via a first-order pathway, accompanied by a small decrease in unit-cell volume (0.2%). Both phases coexist up to 194 GPa. Ab initio random structure search calculations confirm that the hcp phase is the only candidate for another polymorph of Sn occurring in this pressure range, and predict a return to the bcc structure above 1.3 TPa.