Nonmetal - Metallic Allotropes

Metallic Allotropes

Among the nonmetals, many possess metallic allotropes under high pressure, while some metals may exist in nonmetallic allotropes:

Element Period Group Classification Allotropes
Hydrogen 1 1 other nonmetal Metallic hydrogen forms at 260–270 GPa at 295 K and converts back to molecular hydrogen at 200 GPa.
Helium 1 18 noble gas Metallic helium is predicted to occur around 100 Mbar (10 TPa) at low temperatures and 40 Mbar (4 TPa) at high temperatures.
Boron 2 13 metalloid Common allotropes of boron have bandgaps of approximately 2 eV, but a high-pressure superconducting phase occurs at 160 GPa and 250 GPa at 4 and 11 K.
Carbon 2 14 other nonmetal A metallic allotrope of carbon has been hypothesized to occur at 1.1 TPa.
Nitrogen 2 15 other nonmetal There has been some theoretical consideration of a high-pressure metallic allotrope. Despite a calculated transition at 100 GPa, experiments up to 180 GPa failed to detect this.
Oxygen 2 16 other nonmetal Metallic oxygen has been observed at pressures over 96 GPa, and is superconducting at low temperatures.
Fluorine 2 17 halogen The metallization pressure of solid fluorine is expected to exceed 200 GPa.
Neon 2 18 noble gas First-principle calculations estimate that the band gap of neon might close at 142 TPa and metallization may occur at 176 TPa.
Aluminium 3 13 post-transition metal The structure of clusters of aluminium atoms sandwiched among other elements can be extended to hypothesize a nonmetallic "β-aluminium" allotrope; it is not known whether it can physically exist.
Silicon 3 14 metalloid Under increasing pressure silicon transforms from a cubic diamond structure to a β-tin (11–12 GPa), primitive hexagonal (13–16 GPa), hexagonal-close-packed (37–40 GPa), and face-centered cubic phases (78 GPa). Three of these phases are metallic.
Phosphorus 3 15 other nonmetal Two allotropes of phosphorus at atmospheric pressure have sometimes been called metallic – "α-metallic" (violet or Hittorf's phosphorus) and "β-metallic" or black phosphorus. Violet and black phosphorus have bandgaps of 1.5 and 0.34 eV, respectively. Black phosphorus metallizes at 1.7 GPa by bandgap closure without a structural transition.
Sulfur 3 16 other nonmetal Sulfur undergoes transitions to two superconducting metal phases, at roughly 90 GPa and 200 GPa; the first of these has an incommensurate crystal structure.
Chlorine 3 17 halogen Chlorine was estimated to undergo transition to a metal at 67 GPa; this was confirmed, but at higher pressures.
Argon 3 18 noble gas As of 2009, metallization of argon, predicted to occur at very high pressures, has not been observed.
Gallium 4 13 post-transition metal The orthorhombic α-phase of gallium includes a short covalent bond between two of eight atoms of the unit cell, and has a "deep minimum in the electronic density of states at the Fermi energy"; thus it can be called a "metallic molecular crystal" or an "inorganic polymer". These properties are absent in the metallic Ga-II and β-Ga states.
Germanium 4 14 metalloid Germanium undergoes a semiconductor to metal transition at 11 GPa.
Arsenic 4 15 metalloid
Selenium 4 16 other nonmetal Selenium undergoes a semiconductor to metal transition at 20 GPa.
Bromine 4 17 halogen Bromine undergoes a semiconductor to metal transition at 100 GPa.
Krypton 4 18 noble gas Metallization of krypton was predicted to occur at 316 GPa.
Indium 5 13 post-transition metal
Tin 5 14 post-transition metal Of the two common allotropes of tin at room temperature and pressure, white tin (β-tin) is metallic, but gray tin (α-tin) is not. Gray tin is more stable at colder temperatures.
Antimony 5 15 metalloid
Tellurium 5 16 metalloid Tellurium undergoes a semiconductor to metal transition at 4 GPa.
Iodine 5 17 halogen Iodine undergoes a semiconductor to metal transition at 17 GPa.
Xenon 5 18 noble gas Xenon undergoes a semiconductor to metal transition at 160 GPa.
Thallium 6 13 post-transition metal
Lead 6 14 post-transition metal
Bismuth 6 15 post-transition metal
Polonium 6 16 post-transition metal
Astatine 6 17 halogen
Radon 6 18 noble gas

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