Background of Solid Hydrogen

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     H₂ is one of the most extensively studied form of matter that exists (e.g. [7], [8]) because many of its important properties can be predicted theoretically from first principles, and because these results can be compared with experimental data. The theory of H₂ has often functioned as a test bed of general theories of atomic and molecular behavior. Despite this work, the common mechanical properties of solid molecular H₂ are beyond calculation, in part due to the non-ideality of the macroscopic crystal.

        The description of H₂ is complicated by the existence of macroscopic crystals characterized by different nuclear spin and rotational states; para-H₂ has its nucleon spins antialigned with J=0 rotation, and ortho-H₂ has nucleon spins aligned with J=1 rotation. The stable relative concentration of these states is temperature dependent; 75% of H₂ is ortho-H₂ at room temperature (defined as "normal" H₂) whereas equilibrium solid H₂ is almost pure para-H₂. Most of the physical properties of H₂ are mildly dependent on the relative spin state mixture, but there is a significant difference between the specific heats and thermal conductivities of normal and para-H₂ in some temperature ranges [9]. The conversion from normal to para-H₂ is slow, with a time constant on the order of hundreds of hours. Rotational diffusion is a significant phenomenon also. The conversion rate of ortho to para-H₂ has implications for any experiment forming solid H₂ because cooling a room temperature H₂ gas must either catalyze the spin transformation or absorb the large heat of conversion as the solid is formed.

        H₂ has a triple point temperature of 14 K and a triple point pressure of 0.070 atm (54 torr). Solid H₂ forms when liquid H₂ is cooled much below 14 K (supercooling usually occurs). Condensation from the vapor to the solid is relatively slow because of these low pressures.

        Solid H₂ has many peculiar properties as a result of its single proton and electron atoms. H₂ is highly reactive at room temperature and above. Bulk solid H₂, when grown from a melt in equilibrium with its vapor pressure forms as a hcp lattice for both ortho and para species [8]. The description of the crystal structure is complicated, however, because an fcc lattice is stable at temperatures below 3 K, and can occur at higher temperatures for a solid grown from a gas [7],[8]. Para-H₂ is always most stable as an hcp lattice, whereas ortho-H₂ undergoes a phase transition at low temperatures. The solid is a translational quantum solid, where the atoms are not localized at T = 0 K,[8]. It is also highly compressible - by 50% at 10 kbar. The density of solid H₂ and its isotopes are low (0.088, 0.20 and 0.31 g/cm³ for H₂, D₂ and T₂, respectively).

        Past work has centered on microstructural and thermodynamic properties. The microscopic properties are both calculated and derived from macroscopic experiments. The intermolecular interactions of vdW compounds, mixtures, and impurities with vdW solids are by definition small compared with those of common materials. The relative magnitudes that are involved are illustrated by comparing the approximate binding energy of an H₂-H₂ cryogenic dimer - 2.9 cm^-1 [10] with the binding energy of H₂ itself - 104 kcal/mole, or 36,400 cm^-1. As a van der Waals solid, the attraction between H₂ molecules is weak and solid condensations will tend not to form a stable surface layer or hard particles. Because H₂ tends to be very pure when used, it usually exhibits significant supercooling when approaching phase changes.

        Some pertinent, but obscure details of solid H₂: Dislocations and packing defects are the primary defects in single crystals and polycrystalline samples (with relatively large grains) of pure hydrogen. The maximum density of dislocations in parahydrogen is of the order of 10¹⁰ cm^-2, whereas in normal hydrogen it can be an order of magnitude higher. In well-annealed single crystals dislocation densities can be decreased to values as low as 10² - 10³ cm^-2. [11]

        Real Crystal Effects. Vacancies in solid H₂ are equilibrium lattice defects and their concentration is unambiguously determined by temperature and pressure. According to various estimates, at the triple point (T = 13.81 K) of H₂ the concentration of vacancies is from 0.1 - 0.01 %, decreasing rapidly with decreasing temperature. Dislocations and packing defects are the main defects in single crystals and polycrystalline samples (with respectively large grains) of pure H₂. The maximum density of dislocations in para-H₂ is on the order of 10¹⁰ cm^-2, whereas in normal H₂ it can be an order of magnitude higher. In well-annealed single crystals the density of dislocations can be decreased to values perhaps as low as 10² - 10³ cm^-2 [11]. One would thus expect diffusion in solid H₂ to be strongly dependent on the history and preparation of the sample.

        Quantitative research on the recrystallization rate of H₂ is lacking. Experiments at the Institute for Low Temperature (ILT, Kharkov, Ukraine) have shown that the recrystallization rate of H₂ must be relatively large, because fine-grained H₂ samples could not be created [11]. A sample of solid H₂ grown with 1 cm dimensions over one hour gave grain dimensions within the crystal of about 1 mm. Under slower growth rates (hours) 1 cm³ samples became single crystals.

        A summary of the known engineering properties of solid cryogenic molecular H₂ was compiled by Dr. Bates [12] and is published on the Thoughtventions web site,  in contrast to more standard thermodynamic properties [13]. Major compilations of the properties of H₂ have been published by NBS (now NIST) [13] and the Institute of Low Temperature (ILT) in Kharkov, Ukraine [14].

Please email Steve Bates at thought@Thoughtventions.com to discuss this research.

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Last updated: July 2015