The primary purpose of this paper is to stimulate theoretical predictions of how to retain metastably hydrogenous materials made at high pressure P on release to ambient. Ultracondensed metallic hydrogen has been made at high pressures in the fluid and reported made probably in the solid. Because the long quest for metallic hydrogen is likely to be concluded in the relatively near future, a logical question is whether another research direction, comparable in scale to the quest for metallic H, will arise in high pressure research. One possibility is retention of metastable solid metallic hydrogen and other hydrogenous materials on release of dynamic and static high pressures P to ambient. If hydrogenous materials could be retained metastably on release, those materials would be a new class of materials for scientific investigations and technological applications. This paper is a review of the current situation with the synthesis of metallic hydrogen, potential technological applications of metastable metallic H and other hydrogenous materials at ambient, and general background of published experimental and theoretical work on what has been accomplished with metastable phases in the past and thus what might be accomplished in the future. 1 Metastable Ultracondensed Solid Hydrogenous Materials W. J. Nellis Harvard University, Department of Physics, Cambridge MA 02138, USA The primary purpose of this paper is to stimulate theoretical predictions of how to retain metastably hydrogenous materials made at high pressure P on release to ambient. Ultracondensed metallic hydrogen has been made at 140 GPa at finite temperatures T in the fluid. The term “metallic” here means quantum mechanically degenerate. A single sample of ultracondensed hydrogen has been made at an estimated pressure of 495 GPa at 5.5 K. Whether that sample is solid or fluid remains to be demonstrated. Those results imply the long quest for metallic hydrogen is likely to be concluded in the relatively near future. Because the quest for metallic hydrogen has been a major driver of high pressure research for decades, a logical question is whether another research direction, comparable in scale to that quest, will arise in high pressure research in the future. One possibility is retention of metastable solid metallic hydrogen and other hydrogenous materials on release of P and T to ambient. If hydrogenous materials could be retained metastably in the solid on release, those materials would be a new class of materials for scientific investigations and technological applications. This paper is a brief review of the synthesis of metallic hydrogen, potential technological applications of metastable solid metallic H and other hydrogenous materials at ambient, and published experimental and theoretical results as general background for what has been accomplished with metastable phases in the past, which suggests what might be accomplished in the future. 4 systematics of achieving metastable hydrogenous materials is not expected by this author to be particularly fruitful in the foreseeable future. Theoretical predictions are needed to guide experiments. Metastable dense hydrogenous materials have the potential to affect life as we know it in terms of (i) high-Tc superconductors for efficient electrical energy transmission; (ii) quantum solids with unusual physical properties at room temperature, (iii) clean fuels for autos and other vehicles, (iv) energetic propellants for rocket-driven space travel, (v) energy-storage media, (vi) light-weight structural materials, and (vii) nuclear fuel in the isotopic forms of deuterium (D) and tritium (T) for Inertial Confinement Fusion (ICF), a potential source of commercial energy [12]. II. Some Hydrogen Results at High Pressures In 1935 Wigner and Huntington (WH) predicted that ultracondensed H2 would dissociate to H at a density ρ = 0.62 mol H/cm3, P greater than 25 GPa and “very low temperatures” T [5]. Dense diatomic H2 is an insulator with two electrons localized on each H2 molecule. Dense monatomic H has a half-filled electronic energy band and thus is a metal. WH’s prediction initiated a multi-decade search for metallic and superconducting hydrogen and its alloys. Pressures P up to several 100 GPa (100 GPa = 106 bar = 1 Mbar), compressions ρ/ρ0 up to ~10 fold, where ρ is compressed density and ρ0 is initial density, and temperatures T up to several 1000 K in H can be accessed experimentally. Temperatures T of a metal are low or high relative to its Fermi temperature TF [13]. Free-electron TF ∝ ρ2/3. Because hydrogen is very compressible, low degeneracy factors T/T F are readily 8 IV.1 Electronic band structure and H metastability The electronic band structure of metallic H at P = 0 has been calculated and prospects for metastability considered [22]. That classic theoretical study found that metallic hydrogen likely crystallizes into anisotropic, filamentary, triangular structures with nearly degenerate energies and two-dimensional periodicity. The lifetime of the likely metastable ground state is finite but its value remains an open question, which is a serious issue for technological applications. The lifetime of a metastable state can probably be increased by application of a pressure that is much lower than the dissociative metallization pressure, which might not be a very low pressure from a practical standpoint. Near-degeneracy of likely structures suggests the possibility that degenerate solid H is highly defected or amorphous. In fact, metallic hydrogen has been predicted to be a liquid near T = 0 K [10]. IV.2 Nitrogen A polymeric phase of nitrogen has been found in which all atoms are connected by single covalent bonds at pressures above 110 GPa and temperatures above 2000 K obtained in a laser-heated diamond anvil cell. At 300 K polymeric nitrogen is metastable down to 42 GPa. Polymeric nitrogen is predicted to have an energy content more than five times greater than existing