Metallic Properties of the Isotopes of Hydrogen

Harvard University, Cambridge MA

Investigators

Abstract

Non-technical Abstract The hydrogen atom is composed of a single proton and a single electron bonded together. The deuterium nucleus is composed of a proton and neutron, so twice the mass of hydrogen. The objective of this research is to study the properties of the hydrogen isotopes: hydrogen, deuterium, and hydrogen deuteride in the metallic phases (tritium is omitted as it is radioactive), collectively referred to as the hydrogens. In general isotopic effects of elements are small, of order a percent or two. In the hydrogens, the mass ratio of D2 to H2 is 200% and the impact on the behavior can be very large. For example, one of the important quantum aspects of hydrogen is the zero-point energy or motion in the condensed matter phase. The consequence is that in the zero temperature limit, the atoms or moleules are not in fixed positions in lattice sites, but have motion around these sites. Such materials have very low temperatures of solidification (molecular hydrogen, about 14 K; deuterium about 18 K). Helium is even more quantum than the molecular hydrogens and does not have a solid state, remaining liquid to T= 0K. Atomic hydrogen itself is predicted to have a liquid ground state. The research team plans to study the effects of high pressure on the hydrogens. The team has already shown experimentally that hydrogen deuteride, which is a stable molecule, becomes unstable at a pressure of ~200 GPa, dissociating and recombining to form a stable mixture of HD, D2 an H2. Thus, in this research the properties of the isotopes at ultra high pressures and low temperatures shall be studied. Technical Abstract The program of research is to study the properties of the hydrogens at pressure where they are metallic. In the research program it has already been shown that hydrogen becomes a metal at ~500 GPa. It is expected that the pressure will be higher for deuterium as, due to its larger mass, molecular deuterium will have smaller zero-point energy and a larger binding energy. HD should have many surprises as above ~200 GPa it becomes a mixture of 0.5 HD and 0.25 (H2+D2). It may require a pressure of 500 GPa to dissociate the H2 molecules, but the HD may become unstable at still a lower pressure. Surprises are expected in this study. Once the isotopes are metallized, the ground state will be studied by xray analysis. One of the intriguing predictions for hydrogen is that it may be a room temperature superconductor, which will be studied. It has also been predicted to be metastable, i.e. it may remain in the metallic state when the pressure is relieved. The metastability of the isotopes shall be studied by reducing the pressure at low temperature, then raising the temperature until the sample converts back to the molecular phase. It may be that hydrogen is not metastable, but deuterium is, due to its smaller zero point energy. Finally, a pressure scale for ultrahigh pressures will be developed. There is no good scale at 500+GPa. A study of the shift of the fluorescence line in nitrogen vacancy nano-diamond can lead to a new pressure calibration scale. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.