Evidence for a structural phase transition in solid hydrogen at megabar pressures

Abstract

We have studied solid molecular hydrogen at pressures up to 167 GPa (1.67 Mbar) and temperatures from 4.3 to 150 K. We have investigated the phase transition recently observed by Hemley and Mao near 150 GPa and identified as orientational ordering by studying the Raman vibrational and rotational excitations. The phase line determined for this transition differs significantly from that expected for the extension of the low-pressure orientational ordering. We conclude that this transition is to a new highpressure molecular phase which only exists at megabar pressures.

The properties of hydrogen and helium under extreme conditions

“…At the Phase II-III transition, a jump in the intermolecular vibron Lorenzana et al, 1989) and a large increase in absorbance of the IR-active vibron are observed. Also, the number of low-frequency Raman-active modes and possible second Raman vibron indicate that in Phase III, the primitive cell should contain at least four molecules (Goncharov et al, 1998).…”

“…As the pressure is further increased to ∼ 150 GPa, molecular hydrogen undergoes another transition (Hemley Lorenzana et al, 1989) to Phase III. The thermodynamic stability range of Phase III has recently been experimentally demonstrated to extend to pressures over 300 GPa and temperatures up 300 K (Zha et al, 2012).…”

The ruby pressure standard to 150GPa

“…Although Dewaele et al report no evidence of nonhydrostatic stresses in He up to 150 GPa, this was a nonquantitative determination. Lorenzana et al 35 studied the pressure distribution in ͑quasihydrostatic͒ hydrogen at ϳ150 GPa and ϳ77 K and measured a variation of 8 GPa using a distribution of ruby grains. Thus, it is possible that the pressure markers in helium were not all at the same pressure.…”

Evidence of a first-order phase transition to metallic hydrogen

“…Pathway I is at lower temperatures in the solid state; metallization requires static pressures not yet achieved on hydrogen in a diamond-anvil cell (DAC). However, four phases have been identified along this pathway, named I, II, III [28][29][30], and IV [9,10]. These are structural phase transitions of orientational-order of the molecules in which the solid remains insulating.…”