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,, and. 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 units, and can be a superconductor at 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,, or 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 substructure is linear.
Trihydrogen cation| would form triangular structures in the hypothetical.

Compounds

When sodium hydride is compressed with hydrogen, and 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.
formulanametemperature
°C		pressure
GPa		crystal structurespace groupa Åbcβcell volumeformulae
per
unit cell		Tc KCommentrefs
lithium dihydride27130
Lithium hexahydride
Lithium heptahydride
sodium trihydrideorthorhombicCmcm3.332 Å6.354 Å4.142 Å9087.694
sodium heptahydridemonoclinicCc6.993.5975.54169.465130.5
50022double hexagon
600121
RbH9−x10Cccm
RbH9−xCm
strontium hexahydridepseudo cubicPm'msemiconductor
metallize > 220 GPa
tristrontium tridecahydrideC2/m
strontium docosahydride138triclinicP1
Barium dodecahydride75pseudo cubic5.435.415.3739.4820K
iron pentahydride120066tetragonalI4/mmm
Sulfur trihydride25150cubicIm'm203K
Selenium trihydride10
yttrium tetrahydride700160I4/mmm
yttrium hexahydride700160Im-3m224
yttrium nonahydride400237P63/mmc243
caesium heptahydridetetragonalP4/nmm
CsH15+xtriclinicP1
Lanthanum decahydride1000170cubicFmm5.095.095.091324250K
Lanthanum decahydride25121HexagonalRm3.673.678.831
Lanthanum undecahydride2150130-160TetragonalP4/nmm168
Lanthanum dodecahydrideCubicinsulating
Lanthanum heptahydride25109monoclinicC2/m6.443.83.6913563.92
Cerium nonahydride93hexagonalP63/mmc3.7115.54333.053100K
Cerium decahydrideFm'm115K
Praseodymium nonahydride90-140P63/mmc3.605.4761.555K 9K
Praseodymium nonahydride120F43m4.9812469K
Neodymium tetrahydride85-135tetragonalI4/mmm2.82345,7808
Neodymium heptahydride85-135monoclinicC2/c3.31776.2525.70789.354
Neodymium nonahydride110-130hexagonalP63/mmc3.4585.935
europium tetrahydride50-130I4/mmm
octaeuropium hexatetracontahydride1600130cubicPm'n5.865
Europium nonahydride86-130cubicF3''m
Europium nonahydride>130hexagonalP''63/mmc
Thorium tetrahydride86I4/mmm2.9034.42157.232
Thorium tetrahydride88trigonalP3215.5003.2986.18
Thorium tetrahydrideorthorhombicFmmm
Thorium hexahydride86-104Cmc2132.36
Thorium nonahydride2100152hexagonalP63/mmc3.7135.54166.20
Thorium decahydride180085-185cubicFm''m''5.29148.0161
Thorium decahydride<85Immm5.3043.2873.64774.03
Uranium heptahydride200063fccP63/mmc
Uranium octahydride3001-55fccFmm
Uranium nonahydride40-55fccP63/mmc