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For the reducing agent known as Superhydride, see Lithium triethylborohydride.

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.[1]

Polyhydrides are only known to be stable under high pressure.[1]

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.[1]

Structures

Thumb
Unit cell diagram showing the structure of NaH7, which contains H−3 complexes. The coloured balls in the isosurface, plotted at the level of 0.07 electrons*Å−3. One of H2 molecules is bonded to a hydrogen atom in the NaH unit with a bond length of 1.25 Å, forming a H−3 linear anion.

The polyhydrides of alkaline earth and alkali metals contain cage structures. Also hydrogen may be clustered into H−, H−3, or H2 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.[1] The H−3 substructure is linear.[2]

H+3 would form triangular structures in the hypothetical H5Cl.[2]

Compounds

Summarize
Perspective

When sodium hydride is compressed with hydrogen, NaH3 and NaH7 form. These are formed at 30 GPa and 2,100 K.[2]

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.[3]

More information formula, name ...
formula name temperature

°C

 pressure

GPa

 crystal structure space group a Å b c β cell volume formulae

per unit cell

 Tc K Comment refs
LiH2 lithium dihydride 27 130 [4]
LiH6 Lithium hexahydride [1]
LiH7 Lithium heptahydride [1]
NaH3 sodium trihydride orthorhombic Cmcm 3.332 Å 6.354 Å 4.142 Å 90 87.69 4 [2]
NaH7 sodium heptahydride monoclinic Cc 6.99 3.597 5.541 69.465 130.5 [2]
CaHx 500 22 double hexagon [5]
CaHx 600 121 [5]
RbH9−x 10 Cccm [6]
RbH9−x Cm
SrH6 pseudo cubic Pm3m semiconductor

metallize > 220 GPa

 [7]
Sr3H13 C2/m [7]
SrH22 138 triclinic P1 [7]
BaH12 Barium dodecahydride 75 pseudo cubic 5.43 5.41 5.37 39.48 20K [8][9]
FeH5 iron pentahydride 1200 66 tetragonal I4/mmm [1]
H3S Sulfur trihydride 25 150 cubic Im3m 203K [10]
H3Se Selenium trihydride 10 [11]
YH4 yttrium tetrahydride 700 160 I4/mmm [12]
YH6 yttrium hexahydride 700 160 Im-3m 224 [12][13][14]
YH9 yttrium nonahydride 400 237 P63/mmc 243 [12]
CsH7 tetragonal P4/nmm [6]
CsH15+x triclinic P1 [6]
LaH10 Lanthanum decahydride 1000 170 cubic Fm3m 5.09 5.09 5.09 132 4 250K [15][16]
LaH10 Lanthanum decahydride 25 121 Hexagonal R3m 3.67 3.67 8.83 1 [15]
LaD11 Lanthanum undecahydride 2150 130-160 Tetragonal P4/nmm 168 [16]
LaH12 Lanthanum dodecahydride Cubic insulating [16]
LaH7 Lanthanum heptahydride 25 109 monoclinic C2/m 6.44 3.8 3.69 135 63.9 2 [15]
CeH9 Cerium nonahydride 93 hexagonal P63/mmc 3.711 5.543 33.053 100K [17]
CeH10 Cerium decahydride Fm3m 115K [18]
PrH9 Praseodymium nonahydride 90-140 P63/mmc 3.60 5.47 61.5 55K 9K [19][20]
PrH9 Praseodymium nonahydride 120 F43m 4.98 124 69K [19]
NdH4 Neodymium tetrahydride 85-135 tetragonal I4/mmm 2.8234 5,7808 [21]
NdH7 Neodymium heptahydride 85-135 monoclinic C2/c 3.3177 6.252 5.707 89.354 [21]
NdH9 Neodymium nonahydride 110-130 hexagonal P63/mmc 3.458 5.935 [21]
EuH4 50-130 I4/mmm [22]
Eu8H46 1600 130 cubic Pm3n 5.865 [22]
EuH9 Europium nonahydride 86-130 cubic F43m [22]
EuH9 Europium nonahydride >130 hexagonal P63/mmc [22]
ThH4 Thorium tetrahydride 86 I4/mmm 2.903 4.421 57.23 2 [3]
ThH4 Thorium tetrahydride 88 trigonal P321 5.500 3.29 86.18 [3]
ThH4 Thorium tetrahydride orthorhombic Fmmm [3]
ThH6 Thorium hexahydride 86-104 Cmc21 32.36 [3]
ThH9 Thorium nonahydride 2100 152 hexagonal P63/mmc 3.713 5.541 66.20 [3]
ThH10 Thorium decahydride 1800 85-185 cubic Fm3m 5.29 148.0 161 [3]
ThH10 Thorium decahydride <85 Immm 5.304 3.287 3.647 74.03 [3]
UH7 Uranium heptahydride 2000 63 fcc P63/mmc [23]
UH8 Uranium octahydride 300 1-55 fcc Fm3m [23]
UH9 Uranium nonahydride 40-55 fcc P63/mmc [23]
Close

Predicted

Using computational chemistry many other polyhydrides are predicted, including LiH8,[24] LiH9,[25] LiH10,[25] CsH3,[26] KH5, RbH5,[27] RbH9,[24] NaH9, BaH6,[27] CaH6,[28] MgH4, MgH12, MgH16,[29] SrH4,[30] SrH10, SrH12,[24] ScH4, ScH6, ScH8,[31] YH4 and YH6,[32] YH24, LaH8, LaH10,[33] YH9, LaH11, CeH8, CeH9, CeH10,[34] PrH8, PrH9,[35] ThH6, ThH7 and ThH10,[36] U2H13, UH7, UH8, UH9,[23] AlH5,[37] GaH5, InH5,[24] SnH8, SnH12, SnH14,[38] PbH8,[39] SiH8 (subsequently discovered),[24] GeH8,[40] (although Ge3H11 may be stable instead)[41] AsH8, SbH4,[42] BiH4, BiH5, BiH6,[43] H3Se,[44] H3S,[45] Te2H5, TeH4,[46] PoH4, PoH6,[24] H2F, H3F,[24] H2Cl, H3Cl, H5Cl, H7Cl,[47] H2Br, H3Br, H4Br, H5Br, H5I,[24] XeH2, XeH4.[48]

Among the transition elements, VH8 in a C2/m structure around 200 GPa is predicted to have a superconducting transition temperature of 71.4 K. VH5 in a P63/mmm space group has a lower transition temperature.[49]

Properties

Summarize
Perspective

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.[35][50][51] 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.[52]

For example, lithium hexahydride is predicted to lose all electrical resistance below 38 K at a pressure of 150 GPa. The hypothetical LiH8 has a predicted superconducting transition temperature at 31 K at 200 GPa.[53] MgH6 is predicted to have a Tc of 400 K around 300 GPa.[54] CaH6 could have a Tc of 260 K at 120 GPa. PH3 doped H3S is also predicted to have a transition temperature above the 203 K measured for H3S (contaminated with solid sulfur).[55] Rare earth and actinide polyhydrides may also have highish transition temperatures, for example, ThH10 with Tc = 241 K.[36] UH8, which can be decompressed to room temperature without decomposition, is predicted to have a transition temperature of 193 K.[36] AcH10, if it could be ever made, is predicted to superconduct at temperatures over 204 K, and AcH10 would be similarly conducting under lower pressures (150 GPa).[56]

H3Se actually is a van der Waals solid with formula 2H2Se·H2 with a measured Tc of 105 K under a pressure of 135 GPa.[11]

Ternary superhydrides

Ternary superhydrides open up the possibility of many more formulas.[57] For example, Li2MgH16 may also be superconducting at high temperatures (200 °C).[58] A compound of lanthanum, boron and hydrogen is speculated to be a "hot" superconductor (550 K).[59][60] Elements may substitute for others and so modify the properties eg (La,Y)H6 and (La,Y)H10 can be made to have a slightly higher critical temperature than YH6 or LaH10.[61]

See also

References

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