A polyhydride or superhydride is a compound that contains an abnormally large amount of

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

. This can be described as high hydrogen

stoichiometry Stoichiometry () is the relationships between the masses of reactants and Product (chemistry), products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must ...

. Examples include

iron pentahydride Iron pentahydride is a polyhydride, superhydride compound of iron and hydrogen, stable under high pressures. It is important because it contains atomic hydrogen atoms that are not bonded into smaller molecular clusters, and may be a superconductor ...

, , and . By contrast, the more well known

lithium hydride Lithium hydride is an inorganic compound with the formula Li H. This alkali metal hydride is a colorless solid, although commercial samples are grey. Characteristic of a salt-like (ionic) hydride, it has a high melting point, and it is not solub ...

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 Metallic hydrogen is a phase of hydrogen in which it behaves like an electrical conductor. This phase was predicted in 1935 on theoretical grounds by Eugene Wigner and Hillard Bell Huntington. At high pressure and temperatures, metallic hydr ...

, but can be made under lower pressures. One possibility is that they could be superconductors.

Hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

under high pressures forms units, and can be a superconductor at and a pressure of 1.5 million atmospheres (152 GPa).

Structures

The polyhydrides of

alkaline earth The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are al ...

and

alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...

s contain cage structures. Also hydrogen may be clustered into , , or units. Polyhydrides of

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...

s may have the hydrogen atoms arranged around the metal atom. Computations suggest that increasing hydrogen levels will reduce the dimensionality of the metal arrangement, so that layers form separated by hydrogen sheets. The substructure is linear. would form triangular structures in the hypothetical .

Compounds

When

sodium hydride Sodium hydride is the chemical compound with the empirical formula Na H. This alkali metal hydride is primarily used as a strong yet combustible base in organic synthesis. NaH is a saline (salt-like) hydride, composed of Na+ and H− ions, in co ...

is compressed with hydrogen, and form. These are formed at 30 GPa and 2,100 K. Heating and compressing a metal with

ammonia borane Ammonia borane (also systematically named ammoniotrihydroborate), also called borazane, is the chemical compound with the formula . The colourless or white solid is the simplest molecular boron-nitrogen-hydride compound. It has attracted attention ...

avoids using bulky hydrogen, and produces

boron nitride Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula B N. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexago ...

as a decomposition product in addition to the polyhydride.

Predicted

Using

computational chemistry Computational chemistry is a branch of chemistry that uses computer simulations to assist in solving chemical problems. It uses methods of theoretical chemistry incorporated into computer programs to calculate the structures and properties of mol ...

many other polyhydrides are predicted, including , , , , , , , , , , , , , , , , , , , and , , , , , , , , , , , , and , , , , , , , , , , , , (subsequently discovered), , (although may be stable instead) , , , , , , , , , , , , , , , , , , , , , , , . Among the transition elements, in a ''C''2/''m'' structure around 200 GPa is predicted to have a superconducting transition temperature of 71.4 K. in a ''P''6₃/''mmm'' space group has a lower transition temperature.

Properties

Superconduction

Under suitably high pressures polyhydrides may become

superconducting Superconductivity is a set of physical properties observed in superconductors: materials where electrical resistance vanishes and magnetic fields are expelled from the material. Unlike an ordinary metallic conductor, whose resistance decreases g ...

. Characteristics of substances that are predicted to have high superconducting temperatures are a high phonon frequency, which will happen for light elements, and strong bonds. Hydrogen is the lightest and so will have the highest frequency of vibration. Even changing the isotope to

deuterium Deuterium (hydrogen-2, symbol H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen; the other is protium, or hydrogen-1, H. The deuterium nucleus (deuteron) contains one proton and one neutron, whereas the far more c ...

will lower the frequency and lower the transition temperature. Compounds with more hydrogen will resemble the predicted metallic hydrogen. However, superconductors also tend to be substances with high symmetry and also need the electrons not to be locked into molecular subunits, and require large numbers of electrons in states near the

Fermi level The Fermi level of a solid-state body is the thermodynamic work required to add one electron to the body. It is a thermodynamic quantity usually denoted by ''μ'' or ''E''F for brevity. The Fermi level does not include the work required to re ...

. There should also be electron-phonon coupling which happens when the electric properties are tied to the mechanical position of the hydrogen atoms. The highest superconduction critical temperatures are predicted to be in groups 3 and 3 of the periodic table. Late transitions elements, heavy lanthanides or actinides have extra d- or f-electrons that interfere with superconductivity. For example, lithium hexahydride is predicted to lose all electrical resistance below 38 K at a pressure of 150 GPa. The hypothetical has a predicted superconducting transition temperature at 31 K at 200 GPa. is predicted to have a T_c of 400 K around 300 GPa. could have a T_c of 260 K at 120 GPa. doped is also predicted to have a transition temperature above the 203 K measured for (contaminated with solid sulfur). Rare earth and actinide polyhydrides may also have highish transition temperatures, for example, with T_c = 241 K. , which can be decompressed to room temperature without decomposition, is predicted to have a transition temperature of 193 K. , if it could be ever made, is predicted to superconduct at temperatures over 204 K, and would be similarly conducting under lower pressures (150 GPa). actually is a van der Waals solid with formula with a measured T_c of 105 K under a pressure of 135 GPa.

Ternary superhydrides

Ternary superhydrides open up the possibility of many more formulas. For example, may also be superconducting at high temperatures (200 °C). A compound of lanthanum, boron and hydrogen is speculated to be a "hot" superconductor (550 K). Elements may substitute for others and so modify the properties eg and can be made to have a slightly higher critical temperature than or .

References

{{Reflist Hydrogen compounds High-temperature superconductors