Back in 1935, Eugene Wigner and Hillard Huntington predicted that under high enough pressures solid hydrogen would become metallic, with properties similar to the alkali metals (superconductor and an effective and clean rocket propellant (2,4). In addition, understanding metallic hydrogen could be important for improving energy yields from nuclear fusion, since the nuclei would be closer together in a high-density metallic form of the element than in the molecular form (5). These investigations could also provide new insights into the nature of Jupiter and Saturn, which consist largely of hydrogen under enormous pressure (2,4). Metallic hydrogen could be especially useful if it remains metallic at lower pressures. The stability of metallic hydrogen under these conditions would be analogous to the stability of diamond, which is formed at high temperature and pressure, yet is stable at normal pressures and temperatures, even though it is thermodynamically less stable than graphite.

Edwards and Ashcroft at Cornell University attempted to prepare metallic hydrogen by applying a pressure of 1.5 million atmospheres to solid hydrogen at temperatures below 140 K. Yet even under these extreme conditions, hydrogen did not become metallic (3). Instead, the H₂ molecules form a permanent dipole, with a partial (d) positive charge localized on one H atom and a partial negative charge located on the other:

H^d+..H^d-

The effect of the dipole formation is to prevent solid hydrogen from becoming metallic.

Theoretically, as the pressure increases, the energy required for electrons to escape the H₂ molecule (escape the valence band) and become delocalized throughout the solid (enter the conduction band) becomes smaller. If the pressure could be increased sufficiently, the energy required to jump from the valence band to the conduction band could be reduced to zero (or close to it), and hydrogen would become metallic. On the other hand, if H₂ forms a dipole, the energy required to escape the valence band becomes larger. And if it forms an ionic compound, the energy increases to a point where theory predicts that no amount of pressure can make solid hydrogen metallic. Unfortunately what appears to happen is that the gap between the valence and conduction bands widens with pressure, so that they do not overlap in the range of pressure studied, and hydrogen remains an insulator.

Quantum mechanical calculations by Ashcroft predict that a spontaneous polarization occurs when H₂ has been compressed to about one ninth the volume it occupies at 1 atmosphere, and that this polarization is accompanied by a distortion of the hydrogen nuclei from the ideal lattice sites occupied in the nonpolarized form (3). This suggests that at high enough pressures solid hydrogen becomes completely ionic, consisting of protons and hydride ions; further experiments are necessary to confirm this.

Scientists have already succeeded in making metallic liquid hydrogen at a pressure of 1.5 million atmospheres and 3000 K (5). At that temperature there is enough energy for electrons to jump from the valence band to the conduction band. To this point, no one has succeeded in making solid hydrogen metallic. More experiments involving greater pressures will be done to try to achieve this goal. If no one succeeds, then current theories underlying predictions of a metallic form of solid hydrogen may need to be revised.

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