Orientational phase transitions in hydrogen at megabar pressures

Abstract

We have studied solid molecular hydrogen in various ortho-para concentrations at megabar pressures and down to liquid-helium temperatures. From changes of Raman spectra of rotational and vibrational modes we have identified three new phases. We show evidence for the orientationally ordered phase of parahydrogen at a pressure of 110 GPa (1.1 Mbar) and 8 K and for molecular orientational ordering within the newly discovered hydrogen-^ phase. PACS numbers: 62.50.+p, 64.70.Kb, 67.80.CxSolid hydrogen and deuterium have fascinating phase diagrams which depend on ortho-para concentration, as well as pressure and temperature. At low temperature and pressure, orientational ordering occurs in pure 0-H2 or p-T>2 (species with rotational quantum number 7 = 1) and in ortho-para mixtures rich in the 7 = 1 species. At intermediate pressures, solids of pure p-Hi or 0-D2 (molecules in the spherically symmetric 7=0 state for low pressure) are predicted to order orientationally due to a fundamental lowering of the symmetry of the wave function. This ordering is called the broken-symmetry phase (BSP) transition. At very high pressures, the solids are predicted to metallize due to band overlap. Recently, Hemley and Mao x reported spectroscopic observation of a transition in hydrogen near 150 GPa. Lorenzana, Silvera, and Goettel 2 showed that the high-pressure phase diagram is richer than had been anticipated by demonstrating that this transition is to a new phase, called hydrogen-^ (H-A), which only exists at megabar pressures. Here we present new results for orientational ordering of hydrogen at pressures greater than 100 GPa (1 Mbar). Our results include (l) observation of the BSP transition in p-Hi at 110 GPa, (2) determination of a phase line for orientational ordering of P-H2 in the H-A phase, and (3) demonstration that ortho-para mixtures order within the H-A phase.In order to understand the transitions we have observed, we first review the orientational properties of hydrogen. At low pressures in the solid state, hydrogen molecules are in almost free-rotor states, with spherical harmonics, YJM, as the single-molecule wave functions. Here 7 is the rotational quantum number and M is its projection. At low temperatures, only the 7=0 or 7=1 levels are thermally populated due to the large separation of the rotational energy levels. 3 At low pressure, hydrogen solidifies in the hexagonal-close-packed (hep) structure. 0-H2 molecules are in the p-orbital-like Y\M states and the molecular axes can be ordered in space. At zero pressure 0-H2 orientationally orders into the Pa3 lattice at 2.8 K. 3 In this structure the molecular axes are oriented along the body diagonals of a facecentered-cubic (fee) lattice. This ordering minimizes the anisotropic intermolecular interactions which are dominated by the electric quadrupole-quadrupole interaction.

Simulation and understanding of atomic and molecular quantum crystals

“…Phase I corresponds to the close-packed hcp structure, in which para-H 2 molecules have zero angular momentum and spherically symmetric wave functions (Silvera, 1980). At low temperatures and as pressure is increased, breaking of rotational symmetry eventually occurs and the crystal stabilises in phase II (Lorenzana, Silvera, and Goettel, 1990); the boundary between phases I and II is strongly dependent on isotope type (see Fig. 18), which indicates the presence of important QNE.…”

“…This is the case, for instance, of crystals at extreme thermodynamic conditions (Loubeyre, 1987;Cazorla and Boronat, 2008a;Cazorla and Errandonea, 2014;Cazorla and Boronat, 2015b). A possible solution to overcome this modeling difficulty is to go beyond pairwise additivity, that is, to consider higher order terms in the approximation to the atomic interactions.…”

“…For instance, narrowing (widening) of the peaks in g(r) may be caused by the presence of heavier (lighter) species. Kinetic isotopic effects can also manifest in the thermal expansion of quantum solids (Pamuk et al, 2012;Herrero and Ramírez, 2011a) and corresponding P − T boundaries delimiting the thermodynamic stability regions of different phases (Lorenzana, Silvera, and Goettel, 1990;Goncharov, Hemley, and Mao, 2011).…”

The properties of hydrogen and helium under extreme conditions

“…The structure of Phase II is determined by zero point rotational energy of the molecules. Hence, the phase I-II boundary is sensitive to isotope as well as temperature; at very low temperatures, the transition to the Phase II occurs at 27.8 GPa in o-D 2 (Silvera and Wijngaarden, 1981), 70 GPa in HD (Moshary et al, 1993), and 110 GPa (Lorenzana et al, 1990).…”

Pathways to metallic hydrogen

“…4; a similar diagram exists for parahydrogen. The LP phase (low pressure, sometimes called phase I) is an HCP solid; the BSP (broken symmetry phase), first predicted by Raich and Etters [41], exhibits orientational order of para-hydrogen and has been observed in D 2 , H 2 , and HD (at 28, 110, and 69 GPa, respectively, in the T=0 limit [42][43][44]). …”