Diatomic hydrogen, H2, is a gas at ordinary pressures (~100 Pa) and temperatures (~300 K, 27 oC). When cooled below 33 K (-240 oC) at pressures greater than 1.3 kPa, H2 liquefies and is used industrially.
In 1935, physicists Eugene Wigner and Hillard Huntington predicted that at sufficiently high pressure (and low enough temperature), hydrogen would become a metallic solid consisting of hydrogen atoms. Jupiter, a giant gas planet, is 90% hydrogen that exists as a diatomic gas in its outer atmosphere. Deeper in, the very large mass of the planet’s atmosphere creates high enough pressure to condense the gaseous H2 into a dense fluid near Jupiter’s surface. Even higher pressures exist in its interior. Computer modeling by planetary scientists suggests that in the interior, pressures near 1.93 x 1011 Pa (193 gigapascals) are sufficient to form liquid metallic hydrogen.
On Earth, laboratories achieve ultra- high pressures by using anvil presses. The target substance is compressed by squeezing it between opposing diamond-tipped metal anvils. Recently, researchers at the University of Edinburgh (Scotland) put a small amount of H2 gas between two diamond anvil presses to create ultra-high pressures as high as 384 gigapascals (55 x 10 6 psi). At a pressure of 325 gigapascal the hydrogen became a solid. It is particularly noteworthy that the process was done at about 300 K, close to room temperature. "This is at much higher pressures and much higher temperatures than previous work”, said researcher Phillip Dalladay-Simpson. This is the first time anyone has seen this form of the hydrogen at such a high temperature. This new phase (form) of hydrogen is called Phase V. To determine the structure of Phase V hydrogen, the researchers fired a laser at it and observed the way the wavelength of the light changed. They found that the hydrogen atoms (not diatomic H2 molecules) form alternating layers of orderly and random arrangements. Further study is needed to more fully characterize Phase V. Doing so at such extreme pressures is not easy. For example, conductivity studies must still be done to validate the possible formation of metallic hydrogen. However, the gap between the anvil diamond tips is so small that electrodes used to test conductivity don’t fit into the gap.