Hydrogen is the lightest and most abundant element in the universe, existing primarily as a gas of diatomic molecules (\(\text{H}_2\)) under normal conditions. This colorless, odorless gas is highly combustible. Its behavior under extreme conditions has been a central focus of high-pressure physics for decades. While commonly known in its gaseous and liquid forms, hydrogen can indeed be solidified. The pursuit of solid hydrogen explores exotic states of matter that hold immense scientific and technological potential.
The Conditions for Solid Hydrogen
To create non-metallic solid hydrogen, two conditions must be met: extremely low temperatures and modest pressure. At standard atmospheric pressure, liquid hydrogen becomes a transparent, insulating solid when cooled below its melting point of about 14 Kelvin (approximately \(-259\) degrees Celsius). This cryogenic requirement means the non-metallic solid is far removed from everyday experience. The resulting molecular solid hydrogen is a poor conductor of electricity and heat, confirming its insulating nature and large electronic band gap. This solid is only stable under these intensely cold conditions.
Molecular vs. Atomic Solid Structure
The standard, non-metallic form of solid hydrogen is defined as a molecular solid. In this structure, hydrogen atoms remain tightly bound in pairs, forming distinct \(\text{H}_2\) molecules. These molecules are held in a crystalline lattice by weak Van der Waals forces, and the covalent bond within the molecule remains unbroken. Scientists theorize a different state, the atomic solid, where immense pressure overcomes the strong covalent bond within the \(\text{H}_2\) molecule. This pressure forces the molecule to dissociate, resulting in a lattice of individual hydrogen atoms, which is the prerequisite for the material to exhibit metallic properties.
The Quest for Metallic Hydrogen
The search for metallic hydrogen, often called the “holy grail of high-pressure physics,” began in 1935 when Eugene Wigner and Hillard Bell Huntington predicted its existence. They theorized that sufficient compression would cause a phase transition from an insulating molecular solid to an electrically conductive atomic solid. This conductivity arises when pressure forces the electron orbitals of neighboring atoms to overlap, causing the electrons to become delocalized and flow freely, much like in a typical metal. Achieving this transition requires pressures estimated between 400 to 500 gigapascals (GPa)—millions of times greater than Earth’s atmospheric pressure and significantly higher than the pressure at the Earth’s core. The primary tool used to generate these extreme conditions is the Diamond Anvil Cell (DAC), which presses a tiny sample between the tips of two ultra-hard diamonds.
The quest has been marked by controversies, notably a 2017 claim by a Harvard team to have created solid metallic hydrogen at 495 GPa. The experiment generated discussion regarding the accurate measurement of extreme pressure and the stability of the sample. The sample was lost when the diamond anvils fractured, preventing immediate follow-up testing. The pursuit continues globally using advanced DAC designs and sophisticated spectroscopic techniques to definitively prove the metallic state.
Potential Applications of Solid Hydrogen
If stable, solid metallic hydrogen could be created, its properties would be revolutionary, especially if it proved to be “metastable,” retaining its metallic form after pressure release. The most profound predicted property is high-temperature superconductivity, potentially even at room temperature. A room-temperature superconductor could transform global energy transmission, eliminating energy loss in power grids and enabling highly efficient technologies like magnetic levitation trains and advanced medical imaging devices without costly cryogenic cooling. Another major application is in rocketry, where metallic hydrogen could serve as the most powerful chemical rocket propellant ever conceived. The enormous stored energy released when the material transitions back to a gaseous molecular state would provide a theoretical specific impulse up to three times greater than current chemical fuels. This energy density would drastically reduce the necessary launch mass for spacecraft, making single-stage rockets to orbit or deep-space exploration much more feasible.