X-ray diffraction and equation of state of hydrogen at megabar pressures

Abstract

SOLID hydrogen is predicted^1,2 to become metallic at high pressures. Although metallization was recently reported in high-pressure shock-wave compression experiments using liquid hydrogen³, efforts to understand the high-pressure behaviour of the solid phase have relied mainly on spectroscopic studies in the diamond-anvil cell^4–6 and on ab initio calculations^7–10. Central to these studies is the high-pressure crystal structure—something that is difficult to determine in the diamond-anvil cell—and its pressure dependence. Here we report X-ray diffraction measure-ments of the structure of single-crystal molecular hydrogen at pressures of up to 109 GPa for H₂ and 119 GPa for D₂. From these measurements we deduce the high-pressure equation of state (EOS: the volume–pressure relation). We find that solid hydro-gen is more compressible than previously thought, that the crystal becomes increasingly anisotropic with pressure, and that the difference in EOS between H₂ and D₂ is unexpectedly small. The 'softening' of the EOS relative to previous estimates⁴ increases by about 25% the expected transition pressure to the atomic phase. Our EOS differs significantly from that predicted by ab initio calculations^7,9, indicating that theoretical understand-ing of the behaviour of dense hydrogen remains incomplete.