Solid hydrogen is the

solid Solid is a state of matter where molecules are closely packed and can not slide past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the ...

state of the element

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

. At standard pressure, this is achieved by decreasing the temperature below hydrogen's

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...

of . It was collected for the first time by James Dewar in 1899 and published with the title "Sur la solidification de l'hydrogène" (English: On the freezing of hydrogen) in the '' Annales de Chimie et de Physique'', 7th series, vol. 18, Oct. 1899. 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 , 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. Phase II is a broken-symmetry structure in which the H₂ molecules are no longer able to rotate freely. 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. 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 X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...

provides very limited information about the structures. Nevertheless,

phase transition In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic Sta ...

s 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. 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.

Quantum Monte Carlo Quantum Monte Carlo encompasses a large family of computational methods whose common aim is the study of complex quantum systems. One of the major goals of these approaches is to provide a reliable solution (or an accurate approximation) of the ...

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. On this basis, Phase II is believed to be a molecular structure of ''P''2₁/''c'' symmetry; Phase III is (or is similar to) a structure of ''C''2/''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.

References

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External links

Properties of solid hydrogen at very low temperatures
(2001)
Solid hydrogen experiments for atomic propellants
Hydrogen physics Solid-state chemistry Cryogenics Ice fr:Hydrogène solide

Molecular solid hydrogen

See also

References

Further reading

External links