Magnesium hydride is the chemical compound with the molecular formula MgH<sub>2</sub>. It contains 7.66% by weight of hydrogen and has been studied as a potential hydrogen storage medium.
For comparison, one cubic meter can contain 45 kg of hydrogen pressurized at 700 atm, 70 kg of liquid hydrogen, or up to 106 kg of hydrogen bound in magnesium hydride.
Magnesium hydride is also investigated for use in thermobaric weapons and incendiary weapons, standalone or as a mixture with a solid oxidizer; China tested a (non-nuclear) "hydrogen bomb" using the substance. It can be also used in emulsion explosives as a source of bubbles and additional fuel. It can be added to improve heat release of aluminized explosive compositions and to improve burn rate of propellants.
In 1951 preparation from the elements was first reported involving direct hydrogenation of Mg metal at high pressure and temperature (200 atmospheres, 500 ðC) with MgI<sub>2</sub> catalyst:
Lower temperature production from Mg and H<sub>2</sub> using nanocrystalline Mg produced in ball mills has been investigated. Other preparations include:
The room temperature form ñ-MgH<sub>2</sub> has a rutile structure. There are at least four high pressure forms: ó-MgH<sub>2</sub> with ñ-PbO<sub>2</sub> structure, cubic ò-MgH<sub>2</sub> with Pa-3 space group, orthorhombic HP1 with Pbc2<sub>1</sub> space group and orthorhombic HP2 with Pnma space group. Additionally a non stoichiometric MgH<sub>(2-ô)</sub> has been characterised, but this appears to exist only for very small particles<br /> (bulk MgH<sub>2</sub> is essentially stoichiometric, as it can only accommodate very low concentrations of H vacancies).
The bonding in the rutile form is sometimes described as being partially covalent in nature rather than purely ionic; charge density determination by synchrotron x-ray diffraction indicates that the magnesium atom is fully ionised and spherical in shape and the hydride ion is elongated. Molecular forms of magnesium hydride, MgH, MgH<sub>2</sub>, Mg<sub>2</sub>H, Mg<sub>2</sub>H<sub>2</sub>, Mg<sub>2</sub>H<sub>3</sub>, and Mg<sub>2</sub>H<sub>4</sub> molecules identified by their vibrational spectra have been found in matrix isolated samples at below 10 K, formed following laser ablation of magnesium in the presence of hydrogen. The Mg<sub>2</sub>H<sub>4</sub> molecule has a bridged structure analogous to dimeric aluminium hydride, Al<sub>2</sub>H<sub>6</sub>.
MgH<sub>2</sub> readily reacts with water to form hydrogen gas:
At 287 ðC it decomposes to produce H<sub>2</sub> at 1 bar pressure. The high temperature required is seen as a limitation in the use of MgH<sub>2</sub> as a reversible hydrogen storage medium: