Solid hydrogen

Solid hydrogen is the solid state of the element hydrogen, achieved by decreasing the temperature below hydrogen's melting point of 14.01 K (−259.14 °C). It was collected for the first time by James Dewar in 1899 and published with the title "Sur la solidification de l'hydrogène" in the Annales de Chimie et de Physique, 7th series, vol.18, Oct. 1899.[1][2] Solid hydrogen has a density of 0.086 g/cm3 making it one of the lowest density solids.

Molecular solid hydrogen

At low temperatures and at pressures up to around 400 GPa, hydrogen forms a series of solid phases formed from discrete H2 molecules. Phase I occurs at low temperatures and pressures, and consists of a hexagonal close-packed array of freely rotating H2 molecules. Upon increasing the pressure at low temperature, a transition to Phase II occurs at up to 110 GPa.[3] Phase II is a broken-symmetry structure in which the H2 molecules are no longer able to rotate freely.[4] If the pressure is further increased at low temperature, a Phase III is encountered at about 160 GPa. Upon increasing the temperature, a transition to a Phase IV occurs at a temperature of a few hundred Kelvin at a range of pressures above 220 GPa.[5][6]

Identifying the atomic structures of the different phases of molecular solid hydrogen is extremely challenging, because hydrogen atoms interact with X-rays very weakly and only small samples of solid hydrogen can be achieved in diamond anvil cells, so that X-ray diffraction and neutron diffraction provide very limited information about the structures. Nevertheless, phase transitions can be detected by looking for abrupt changes in the Raman spectra of samples. Furthermore, atomic structures can be inferred from a combination of experimental Raman spectra and first-principles modelling.[7] Density functional theory calculations have been used to search for candidate atomic structures for each phase. These candidate structures have low free energies and Raman spectra in agreement with the experimental spectra.[8][9][10] Quantum Monte Carlo methods together with a first-principles treatment of anharmonic vibrational effects have then been used to obtain the relative Gibbs free energies of these structures and hence to obtain a theoretical pressure-temperature phase diagram that is in reasonable quantitative agreement with experiment.[11] On this basis, Phase II is believed to be a molecular structure of P21/c symmetry; Phase III is (or is similar to) a structure of C2/c symmetry consisting of flat layers of molecules in a distorted hexagonal arrangement; and Phase IV is (or is similar to) a structure of Pc symmetry, consisting of alternate layers of strongly bonded molecules and weakly bonded graphene-like sheets.

Metallic solid hydrogen

See Metallic hydrogen.

See also

References

  1. ↑ Correspondence and General A-I DEWAR/Box D I
  2. ↑ Dewar, James (1899). "Sur la solidification de l'hydrogène". Annales de Chimie et de Physique 18: 145–150.
  3. ↑ H.-K. Mao and R. J. Hemley (1994). "Ultrahigh-pressure transitions in solid hydrogen". Rev. Mod. Phys. 66: 671. doi:10.1103/RevModPhys.66.671.
  4. ↑ I. Goncharenko and P. Loubeyre (2005). "Neutron and X-ray diffraction study of the broken symmetry phase transition in solid deuterium". Nature 435: 1206. doi:10.1038/nature03699.
  5. ↑ R. T. Howie, C. L. Guillaume, T. Scheler, A. F. Goncharov and E. Gregoryanz (2012). "Mixed Molecular and Atomic Phase of Dense Hydrogen". Phys. Rev. Lett. 108: 125501. doi:10.1103/PhysRevLett.108.125501.
  6. ↑ M. I. Eremets and I. A. Troyan (2011). "Conductive dense hydrogen". Nat. Materials 10: 927––931. doi:10.1038/nmat3175.
  7. ↑ J. M. McMahon, M. A. Morales, C. Pierleoni and D. M. Ceperley (2012). "The properties of hydrogen and helium under extreme conditions". Rev. Mod. Phys. 84: 1607. doi:10.1103/RevModPhys.84.1607.
  8. ↑ C. J. Pickard and R. J. Needs (2007). "Structure of phase III of solid hydrogen". Nat. Phys. 3: 473–476. doi:10.1038/nphys625.
  9. ↑ C. J. Pickard and R. J. Needs (2009). "Structures at high pressure from random searching". Phys. Status Solidi B 246: 536. doi:10.1002/pssb.200880546.
  10. ↑ C. J. Pickard, M. Martinez-Canales and R. J. Needs (2012). "Density functional theory study of phase IV of solid hydrogen". Phys. Rev. B 85: 214114. doi:10.1103/PhysRevB.85.214114.
  11. ↑ N. D. Drummond, B. Monserrat, J. H. Lloyd-Williams, P. Lopez Rios, C. J. Pickard and R. J. Needs (2015). "Quantum Monte Carlo study of the phase diagram of solid molecular hydrogen at extreme pressures". Nat. Commun. 6: 7794. doi:10.1038/ncomms8794.

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