Polyhydride

A polyhydride or superhydride is a compound that contains an abnormally large amount of hydrogen. This can be described as high hydrogen stoichiometry. Examples include iron pentahydride FeH₅, LiH₆, and LiH₇. By contrast, the more well known lithium hydride only has one hydrogen atom.

Polyhydrides are only known to be stable under high pressure.

Polyhydrides are important because they can form substances with a very high density of hydrogen. They may resemble the elusive metallic hydrogen, but can be made under lower pressures. One possibility is that they could be superconductors. Hydrogen sulfide under high pressures forms SH₃ units, and can be a superconductor at 203 K (−70 °C) and a pressure of 1.5 million atmospheres.