Will Solid Hydrogen Ever Be a Metal?

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tures. The consequences of squeezing solid hydrogen are clearly a good deal more subtle Will solid hydrogen than at first thought, and it turns out that this, the ‘simplest’ of all molecular solids, has a rich and remarkably complex phase ever be a metal? diagram2,4,5 (Fig. 2). In phase I of solid hydrogen, the so-called orientationally disordered state, the individ- Peter P. Edwards and Friedrich Hensel ual molecules execute complete rotational At ultra-high pressures, liquid hydrogen becomes metallic. So should motion in addition to the usual molecular solid hydrogen, yet it stubbornly resists. A newly predicted vibrations. spontaneous asymmetry of molecules in the solid may be the reason. Below a temperature of about 120 K, solid hydrogen undergoes a transition at about n 1926 J. D. Bernal proposed that all The pressure-induced metallization of 1.5 million atmospheres to phase II, a state

matter, when subjected to a high enough solid hydrogen can be viewed in terms of in which the constituent H2 molecules Ipressure, will inevitably become metallic electronic band theory. At low pressures the become ‘frozen’ in a random orientation in — that is, it will be permeated by a sea of solid is an infinite crystalline assembly of iso- the crystal. completely free electrons that conduct elec- lated H2 molecules, all weakly interacting. Phase III is undoubtedly the most tricity easily. The most enticing substance for The electrons are all bound to their mole- intriguing, because elemental hydrogen pressure-induced metallization is, in fact, cules, and need to be freed to conduct elec- remains in this state up to the highest pres- the lightest and supposedly the simplest of all tricity: in band-theory terms, we have a com- sures yet achieved, and still does not become the elements in the periodic table — hydro- pletely filled valence band separated by a very metallic. Perhaps the most remarkable thing gen. Wigner and Huntington in 1935 first large electronic energy gap (>13 eV) from about phase III is the appearance of a striking predicted that molecular diatomic hydrogen the empty conduction band (Fig. 1). Under absorption of infrared radiation, which is would undergo a transition to a metallic state ambient pressure, elemental solid hydrogen completely absent in the isolated molecule at an imposed pressure of about 250,000 could only conduct by the thermal excitation and largely so in the solid at lower pressures atmospheres1; current predictions are in a of large numbers of electrons from the (phases I and II). range close to three million atmospheres. valence band to the conduction band. That Why is the observation of infrared activi- But despite an unrelenting experimental would require enormously high tempera- ty in phase III of dense hydrogen particularly assault at these ultra-high pressures, dense tures. important? Substantial infrared activity can solid hydrogen has so far defied all attempts Solid hydrogen is thus an insulator. But as only arise from molecules with an intrinsic 2 3 at metallization . On page 652 of this issue , the applied pressure increases, H2 molecules electric dipole moment. But free H2 mole- Edwards and Ashcroft describe a new twist to begin to interact more strongly, and the cules have a spatially symmetric electron the continuing saga, which may help explain energy bands widen. In the limit of zero band density distribution between their two pro- the recalcitrance of solid hydrogen towards gap we would have an insulator-to-metal tons, and so do not possess an intrinsic metallization. transition — and metallic hydrogen. dipole moment. All hydrogen molecules Why are we so interested in making Several groups have already succeeded in should therefore show little, if any, infrared metallic hydrogen? It may have exotic compressing solid hydrogen to ultra-high activity in the dense solid. However, in 1994 properties and important applications. For pressures, with an elemental density now strong infrared activity was detected in solid example, it has been predicted to be a room- more than 10 times the room-pressure solid hydrogen at low temperatures and high pres- temperature superconductor; and if it could density2,4. The average density of electrons in sures, occurring only in phase III. Impor- be stably quenched from high pressures to dense solid hydrogen is now two to three tantly, ambient, it would be a highly effective and times that of the excellent metal, aluminium. this fascinating behaviour occurs under exceptionally clean propellant for space And yet, remarkably, solid hydrogen under conditions for which solid hydrogen still travel. The metallization of hydrogen would these crushingly oppressive conditions shows no evidence of metallic behaviour. also provide new insights into the nature of remains a stubborn insulator, with not even a Under ambient pressure, states of H2 that giant planets — Jupiter and Saturn, for hint of metallic behaviour at low tempera- have any dipolar character are much higher example, both contain more than 400 Earth in energy than the unpolarized states. But masses, most of which is hydrogen under Edwards and Ashcroft3 note that, as the band extreme conditions. Edwards and Ashcroft have discovered a Empty

Energy conduction remarkable spontaneous electronic polar- band 200 ization of hydrogen dimer molecules at Phase I ultra-high pressures. In stark contrast to the room-pressure situation, in which a hydro- Filled valence Phase III gen molecule has precisely equivalent elec- band 100 tron (charge) density at both protons, at high Infrared- Temperature (K) Temperature

Insulator–metal transition active pressures the electronic charge piles up pref- Phase II region erentially at just one of the constituent pro- Pressure tons. The hydrogen molecule thus develops a 0 permanent electric dipole moment. This Figure 1 Bands under pressure. With increasing 0.5 1. 0 1. 5 2.0 induced ionic character — in the limit one pressure, and therefore density, the electronic Pressure (million atmospheres) would regard hydrogen as ‘protonium energy bands of solid hydrogen broaden. One Figure 2 A possible phase diagram for dense hydride’, H+H– — may serve to delay, or even assumes that eventually they will overlap, hydrogen. Phase III, which absorbs infrared thwart completely, the long-awaited transi- making a conducting, metallic state; but light strongly, may be characterized by a tion to the metallic state of dense solid spontaneous polarization of the H2 molecules spontaneous polarization of the constituent hydrogen. (Fig. 3) might postpone that, or even prohibit it. hydrogen molecules.

NATURE | VOL 388 | 14 AUGUST 1997 Nature © Macmillan Publishers Ltd 1997 621 news and views

Only time — and pressure — will tell. 1. Wigner, E. & Huntington, H. B. J. Chem. Phys. 3, 764–770 (1935). Peter P. Edwards is in the School of Chemistry, The 2. Silvera, I. F. in Metal–Insulator Transitions Revisited (eds University of Birmingham, Edgbaston, Birmingham Edwards, P. P. & Rao, C. N. R.) 21–42 (Taylor & Francis, London, 1995). B15 2TT, UK. Friedrich Hensel is in the Fachbereich 3. Edwards, B. & Ashcroft, N. W. Nature 388, 652–655 (1997). Chemie, Philipps-Universität Marburg, D-35032 4. Mao, H.-K. & Hemley, R. J. Rev. Mod. Phys. 66, 671–692 (1994). Marburg, Germany. 5. Ashcroft, N. W. Phys. World 8, 43–47 (1995).

Evolutionary biology Even-toed fingerprints on whale ancestry Michel C. Milinkovitch and J. G. M. Thewissen

Figure 3 The newly predicted ground state of molecular hydrogen at ultra-high pressure3. oth morphological1 and molecular2 porting artiodactyl monophyly) and, if cor- Here the electron (charge) cloud preferentially studies indicate that cetaceans rect, would make a cow or a hippopotamus accumulates at just one of the two protons, B(whales, dolphins and porpoises) and more closely related to a dolphin or a whale producing an electric dipole. (The magnitude of artiodactyls (even-toed ungulates, which than to a pig or a camel (Fig. 1). this charge transfer has been exaggerated for include pigs, hippos, camels and ruminants) Although it is compatible with earlier clarity.) The inset shows molecular hydrogen form a clade or monophyletic group — that molecular analyses (for example, refs 4, 5), under normal conditions; here the two protons is, they have a common ancestor that is not the idea that cetaceans are highly derived are surrounded by a symmetrical charge cloud. shared by any other group of mammals. This artiodactyls was first suggested in 1994 on is counter-intuitive, because it implies that a the basis of mitochondrial and nuclear DNA gap of solid hydrogen closes continuously cow is more closely related to a dolphin or a and amino-acid sequences6. The idea was with pressure, there comes a point at which it whale than to a horse, yet it is one of the corroborated by other phylogenetic analyses may be energetically favourable to mix in a best examples of congruence between of DNA sequences7. But the issue is still con- small proportion of these ionic states, result- morphological and molecular estimates of troversial, because the exact means by which ing in a hybridized ground state. The pres- mammalian phylogeny. molecular sequence data should be analysed sure-induced electric dipole formed on any The molecular analyses of Shimamura remains debated — although analytical set- one of these hydrogen molecules will interact et al.3, reported on page 666 of this issue, tings that are particularly meaningful with with, and be stabilized by, the other remain- further disrupt phylogenetic dogma. respect to phylogenetic inferences can prob- ing molecules in the dense solid. The Indeed, not only do the authors confirm the ably be identified in specific instances8. But, stabilization of this new dipolar state will be close relationship between artiodactyls and basically, many morphologists consider that pressure dependent, and so above a critical cetaceans, but they propose that cetaceans molecular data are necessarily more noisy density a spontaneous permanent electric are deeply nested within the phylogenetic than morphological data. polarization sets in on each molecule (Fig. 3). tree of the artiodactyls. These results strik- The analyses by Shimamura and col- Edwards and Ashcroft calculate the ingly contradict the common interpretation leagues now provide a remarkable example necessary conditions for the evolution of an of the available morphological data (sup- of molecular markers, which should lead induced molecular dipole in hydrogen. They predict the appearance of a spontaneous electronic polarization at a pressure where, experimentally, one sees the onset of the striking infrared activity in hydrogen. So dense hydrogen in phase III is composed of Cetaceans (and Ruminants Hippos and Pigs and Camels and molecular, dipolar hydrogen. There may also mesonychians) anthracotheroids peccaries llamas be a reorientation and displacement of the dipolar molecules as the new, partly ionic state of hydrogen forms. Four-cusped bunodont teet But what of the anticipated transition to Large canine with triangular cross-section metallic hydrogen at even higher densities? It has been suggested that molecular hydrogen Trochleated astragalus may eventually become fully ionic, namely Three-lobed dP/4 + – 6 H H , with enough compression . But from Supraorbital foramen Other their calculations Edwards and Ashcroft mammals find that the system does not seem to be progressing towards a fully ionic state. Paraxony What seems clear, however, is that the Lacrimal extensive on face Incrudal crus breve long presence of even partially ionic character in Three bronchi the ground state of dense solid hydrogen will Penile erection based on smooth muscle act to widen the previously narrowing band gap and hence frustrate the transition to the long-sought metallic state. Will solid hydrogen ever become a metal? A pessimistic Figure 1 Twisted or untied? Shimamura et al.3 propose to attach the lineages of hippos and cetaceans prospect, contrary to Bernal’s optimistic (curved arrows, red) to the ruminant branch on this phylogenetic tree of artiodactyls — a marked 1926 generalization, might be that solid diversion from the traditional view (white branching pattern). The two smaller boxes summarize hydrogen may never achieve metallic status. some of the morphological evidence that disagrees with the new data.