Solid hydrogen

Solid hydrogen is the frozen state of the element hydrogen. Most of the time, hydrogen exists as a gas. However, it can become a solid under specific conditions. This transition occurs by decreasing the temperature below its melting point of 14.01 Kelvin.

Solid hydrogen is quite unique because it has a very low density. Its density is measured at only 0.086 grams per cubic centimeter. This makes it one of the lowest-density solids known to science. Understanding this material helps scientists study how atoms behave under extreme conditions.

Scientists study how hydrogen changes using different phases. A phase is a specific state of matter under certain conditions. At low temperatures and pressures, hydrogen forms molecular solid phases. These phases are made of individual H2 molecules. As you change the pressure, the way these molecules sit and move changes. This process is called a phase transition. Researchers use special tools like diamond anvil cells to create these high-pressure environments. These cells allow scientists to squeeze tiny samples of the material to see how it reacts.

There are several distinct phases of molecular solid hydrogen. Phase I occurs at low temperatures and low pressures. In this phase, the H2 molecules form a hexagonal close-packed array. These molecules are able to rotate freely within their structure. If you increase the pressure at a low temperature, the material moves into Phase II. This phase can occur at pressures up to 110 gigapascals. Phase II is a broken-symmetry structure. In this state, the H2 molecules are no longer able to rotate freely.

Further increases in pressure lead to even more complex structures. Phase III is encountered when the pressure reaches about 160 gigapascals. This phase is believed to have a C2/c symmetry. It consists of flat layers of molecules in a distorted hexagonal arrangement. If the temperature also increases, the material reaches Phase IV. This happens at temperatures of a few hundred Kelvin. Phase IV requires pressures above 220 gigapascals. This phase has a Pc symmetry. It features alternating layers of strongly bonded molecules and weakly bonded, graphene-like sheets.

Identifying these atomic structures is a very difficult task for researchers. Hydrogen atoms interact very weakly with X-rays. Because of this, X-ray diffraction provides only limited information about the structures. Scientists must rely on other methods to detect phase transitions. They look for abrupt changes in the Raman spectra of the samples. Raman spectra is a tool used to observe how molecules vibrate. They also use first-principles modelling to infer what the atomic structures look like. This involves using complex computer calculations to predict how atoms will behave.

This field of study has a long history of discovery. The first collection of solid hydrogen was achieved by James Dewar. He accomplished this in 1899. He published his findings in a journal called the Annales de Chimie et de Physique. His paper was titled "Sur la solidification de l'hydrogène," which means "On the freezing of hydrogen."

Since his discovery, scientists have used advanced methods like Density Functional Theory. They also use Quantum Monte Carlo methods to study the energy of these different structures. These modern tools help create a theoretical phase diagram that matches what scientists see in real experiments.

Solid hydrogen also has important uses in space technology. NASA uses frozen hydrogen in the Wide-field Infrared Survey Explorer. This spacecraft carries an infrared instrument that must stay extremely cold. To keep it cold, engineers use a spherical tank called a cryostat. This tank contains solid hydrogen to maintain the necessary low temperatures.

By studying the properties of solid hydrogen, we gain a better understanding of both deep-space technology and the fundamental laws of physics. The way molecules move and bond under pressure reveals much about the physical world.