# 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 FeH$_5$, LiH$_6$, and LiH$_7$. 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 SH$_3$ units, and can be a superconductor at 203 K (−70 °C) and a pressure of 1.5 million atmospheres.

## Structures

The polyhydrides of alkaline earth and alkali metals contain cage structures. Also hydrogen may be clustered into H$^−$, H−3, or H$_2$ units. Polyhydrides of transition metals 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 H−3 substructure is linear.
H+3 would form triangular structures in the hypothetical H$_5$Cl.

## Compounds

When sodium hydride is compressed with hydrogen, NaH$_3$ and NaH$_7$ form. These are formed at 30 GPa and 2,100 K.
Heating and compressing a metal with ammonia borane avoids using bulky hydrogen, and produces boron nitride as a decomposition product in addition to the polyhydride.
| formula | name | temperature °C | pressure GPa | crystal structure | space group | a Å | b | c | β | cell volume | formulae per unit cell | Tc K | Comment |
| LiH$_2$ | lithium dihydride | 27 | 130 | | | | | | | | | | |
| LiH$_6$ | Lithium hexahydride | | | | | | | | | | | | |
| LiH$_7$ | Lithium heptahydride | | | | | | | | | | | | |
| NaH$_3$ | sodium trihydride | | | orthorhombic | Cmcm | 3.332 Å | 6.354 Å | 4.142 Å | 90 | 87.69 | 4 | | |
| NaH$_7$ | sodium heptahydride | | | monoclinic | Cc | 6.99 | 3.597 | 5.541 | 69.465 | 130.5 | | | |
| CaH$_x$ | | 500 | 22 | double hexagon | | | | | | | | | |
| CaH$_x$ | | 600 | 121 | | | | | | | | | | |
| RbH$_{9−x}$ | | | 10 | | Cccm | | | | | | | | |
| RbH$_{9−x}$ | | | | | Cm | | | | | | | | |
| SrH$_6$ | | | | pseudo cubic | Pm3m | | | | | | | | semiconductor metallize > 220 GPa |
| Sr$_3$H$_{13}$ | | | | | C2/m | | | | | | | | |
| SrH$_{22}$ | | | 138 | triclinic | P1 | | | | | | | | |
| BaH$_{12}$ | Barium dodecahydride | | 75 | pseudo cubic | | 5.43 | 5.41 | 5.37 | | 39.48 | | 20K | |
| FeH$_5$ | iron pentahydride | 1200 | 66 | tetragonal | I4/mmm | | | | | | | | |
| H$_3$S | Sulfur trihydride | 25 | 150 | cubic | Im3m | | | | | | | 203K | |
| H$_3$Se | Selenium trihydride | | 10 | | | | | | | | | | |
| YH$_4$ | yttrium tetrahydride | 700 | 160 | | I4/mmm | | | | | | | | |
| YH$_6$ | yttrium hexahydride | 700 | 160 | | Im-3m | | | | | | | 224 | |
| YH$_9$ | yttrium nonahydride | 400 | 237 | | P6$_3$/mmc | | | | | | | 243 | |
| CsH$_7$ | | | | tetragonal | P4/nmm | | | | | | | | |
| CsH$_{15+x}$ | | | | triclinic | P1 | | | | | | | | |
| LaH$_{10}$ | Lanthanum decahydride | 1000 | 170 | cubic | Fm3m | 5.09 | 5.09 | 5.09 | | 132 | 4 | 250K | |
| LaH$_{10}$ | Lanthanum decahydride | 25 | 121 | Hexagonal | R3m | 3.67 | 3.67 | 8.83 | | | 1 | | |
| LaD$_{11}$ | Lanthanum undecahydride | 2150 | 130-160 | Tetragonal | P4/nmm | | | | | | | 168 | |
| LaH$_{12}$ | Lanthanum dodecahydride | | | Cubic | | | | | | | | insulating | |
| LaH$_7$ | Lanthanum heptahydride | 25 | 109 | monoclinic | C2/m | 6.44 | 3.8 | 3.69 | 135 | 63.9 | 2 | | |
| CeH$_9$ | Cerium nonahydride | | 93 | hexagonal | P6$_3$/mmc | 3.711 | | 5.543 | | 33.053 | | 100K | |
| CeH$_{10}$ | Cerium decahydride | | | | Fm3m | | | | | | | 115K | |
| PrH$_9$ | Praseodymium nonahydride | | 90-140 | | P6$_3$/mmc | 3.60 | | 5.47 | | 61.5 | | 55K 9K | |
| PrH$_9$ | Praseodymium nonahydride | | 120 | | F43m | 4.98 | | | | 124 | | 69K | |
| NdH$_4$ | Neodymium tetrahydride | | 85-135 | tetragonal | I4/mmm | 2.8234 | | 5,7808 | | | | | |
| NdH$_7$ | Neodymium heptahydride | | 85-135 | monoclinic | C2/c | 3.3177 | 6.252 | 5.707 | 89.354 | | | | |
| NdH$_9$ | Neodymium nonahydride | | 110-130 | hexagonal | P6$_3$/mmc | 3.458 | | 5.935 | | | | | |
| EuH$_4$ | | | 50-130 | | I4/mmm | | | | | | | | |
| Eu$_8$H$_{46}$ | | 1600 | 130 | cubic | Pm3n | 5.865 | | | | | | | |
| EuH$_9$ | Europium nonahydride | | 86-130 | cubic | F43m | | | | | | | | |
| EuH$_9$ | Europium nonahydride | | >130 | hexagonal | P6$_3$/mmc | | | | | | | | |
| ThH$_4$ | Thorium tetrahydride | | 86 | | I4/mmm | 2.903 | | 4.421 | | 57.23 | 2 | | |
| ThH$_4$ | Thorium tetrahydride | | 88 | trigonal | P321 | 5.500 | | 3.29 | | 86.18 | | | |
| ThH$_4$ | Thorium tetrahydride | | | orthorhombic | Fmmm | | | | | | | | |
| ThH$_6$ | Thorium hexahydride | | 86-104 | | Cmc2$_1$ | | | | | 32.36 | | | |
| ThH$_9$ | Thorium nonahydride | 2100 | 152 | hexagonal | P6$_3$/mmc | 3.713 | | 5.541 | | 66.20 | | | |
| ThH$_{10}$ | Thorium decahydride | 1800 | 85-185 | cubic | Fm3m | 5.29 | | | | 148.0 | | 161 | |
| ThH$_{10}$ | Thorium decahydride | | <85 | | Immm | 5.304 | 3.287 | 3.647 | | 74.03 | | | |
| UH$_7$ | Uranium heptahydride | 2000 | 63 | fcc | P6$_3$/mmc | | | | | | | | |
| UH$_8$ | Uranium octahydride | 300 | 1-55 | fcc | Fm3m | | | | | | | | |
| UH$_9$ | Uranium nonahydride | | 40-55 | fcc | P6$_3$/mmc | | | | | | | | |

### Predicted

Using computational chemistry many other polyhydrides are predicted, including LiH$_8$, LiH$_9$, LiH$_{10}$, CsH$_3$, KH$_5$, RbH$_5$, RbH$_9$, NaH$_9$, BaH$_6$, CaH$_6$, MgH$_4$, MgH$_{12}$, MgH$_{16}$, SrH$_4$, SrH$_{10}$, SrH$_{12}$, ScH$_4$, ScH$_6$, ScH$_8$, YH$_4$ and YH$_6$, YH$_{24}$, LaH$_8$, LaH$_{10}$, YH$_9$, LaH$_{11}$, CeH$_8$, CeH$_9$, CeH$_{10}$, PrH$_8$, PrH$_9$, ThH$_6$, ThH$_7$ and ThH$_{10}$, U$_2$H$_{13}$, UH$_7$, UH$_8$, UH$_9$, AlH$_5$, GaH$_5$, InH$_5$, SnH$_8$, SnH$_{12}$, SnH$_{14}$, PbH$_8$, SiH$_8$ (subsequently discovered), GeH$_8$, (although Ge$_3$H$_{11}$ may be stable instead) AsH$_8$, SbH$_4$, BiH$_4$, BiH$_5$, BiH$_6$, H$_3$Se, H$_3$S, Te$_2$H$_5$, TeH$_4$, PoH$_4$, PoH$_6$, H$_2$F, H$_3$F, H$_2$Cl, H$_3$Cl, H$_5$Cl, H$_7$Cl, H$_2$Br, H$_3$Br, H$_4$Br, H$_5$Br, H$_5$I, XeH$_2$, XeH$_4$.
Among the transition elements, VH$_8$ in a C2/m structure around 200 GPa is predicted to have a superconducting transition temperature of 71.4 K. VH$_5$ in a P6$_3$/mmm space group has a lower transition temperature.

## Properties
### Superconduction

Under suitably high pressures polyhydrides may become superconducting. 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 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. 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 LiH$_8$ has a predicted superconducting transition temperature at 31 K at 200 GPa. MgH$_6$ is predicted to have a T$_c$ of 400 K around 300 GPa. CaH$_6$ could have a T$_c$ of 260 K at 120 GPa. PH$_3$ doped H$_3$S is also predicted to have a transition temperature above the 203 K measured for H$_3$S (contaminated with solid sulfur). Rare earth and actinide polyhydrides may also have highish transition temperatures, for example, ThH$_{10}$ with T$_c$ = 241 K. UH$_8$, which can be decompressed to room temperature without decomposition, is predicted to have a transition temperature of 193 K. AcH$_{10}$, if it could be ever made, is predicted to superconduct at temperatures over 204 K, and AcH$_{10}$ would be similarly conducting under lower pressures (150 GPa).
H$_3$Se actually is a van der Waals solid with formula 2H$_2$Se·H$_2$ 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, Li$_2$MgH$_{16}$ 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 (La,Y)H$_6$ and (La,Y)H$_{10}$ can be made to have a slightly higher critical temperature than YH$_6$ or LaH$_{10}$.