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 (152 GPa).
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.
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 °Cpressure GPacrystal structurespace groupa Åbcβcell volumeformulae per unit cellTc KCommentrefslithium dihydride27130Lithium hexahydrideLithium heptahydridesodium trihydrideorthorhombicCmcm3.332 Å6.354 Å4.142 Å9087.694sodium heptahydridemonoclinicCc6.993.5975.54169.465130.550022double hexagon600121Na3FeH7tetragonalP42/mnmpentagonal bipyramidal FeH73–Na3CoH6orthorhombicPnmactahedral CoH63–RbH9−x10CccmRbH9−xCmstrontium hexahydridepseudo cubicPmmsemiconductor metallize > 220 GPatristrontium tridecahydrideC2/mstrontium docosahydride138triclinicP1Barium dodecahydride75pseudo cubic5.435.415.3739.4820Kiron pentahydride120066tetragonalI4/mmmSulfur trihydride25150cubicImm203KSelenium trihydride10yttrium tetrahydride700160I4/mmmyttrium hexahydride700160Im-3m224yttrium nonahydride400237P63/mmc243caesium heptahydridetetragonalP4/nmmCsH15+xtriclinicP1Lanthanum decahydride1000170cubicFmm5.095.095.091324250KLanthanum decahydride25121HexagonalRm3.673.678.831Lanthanum undecahydride2150130-160TetragonalP4/nmm168Lanthanum dodecahydrideCubicinsulatingLanthanum heptahydride25109monoclinicC2/m6.443.83.6913563.92Cerium nonahydride93hexagonalP63/mmc3.7115.54333.053100KCerium decahydrideFmm115KPraseodymium nonahydride90-140P63/mmc3.605.4761.555K 9KPraseodymium nonahydride120F43m4.9812469KNeodymium tetrahydride85-135tetragonalI4/mmm2.82345,7808Neodymium heptahydride85-135monoclinicC2/c3.31776.2525.70789.354Neodymium nonahydride110-130hexagonalP63/mmc3.4585.935europium tetrahydride50-130I4/mmmoctaeuropium hexatetracontahydride1600130cubicPmn5.865Europium nonahydride86-130cubicF3mEuropium nonahydride>130hexagonalP63/mmcThorium tetrahydride86I4/mmm2.9034.42157.232Thorium tetrahydride88trigonalP3215.5003.2986.18Thorium tetrahydrideorthorhombicFmmmThorium hexahydride86-104Cmc2132.36Thorium nonahydride2100152hexagonalP63/mmc3.7135.54166.20Thorium decahydride180085-185cubicFmm5.29148.0161Thorium decahydride<85Immm5.3043.2873.64774.03Uranium heptahydride200063fccP63/mmcUranium octahydride3001-55fccFmmUranium nonahydride40-55fccP63/mmc
Predicted Using computational chemistry many other polyhydrides are predicted, including , , , , , , , , , , , , , , , , , , , and , , , , , , , , , , , , and , , , , , , , , , , , , (subsequently discovered), , (although may be stable instead) , , , , , , , , , , , , , , , , , , , , , , , .
Among the transition elements, in a C2/m structure around 200 GPa is predicted to have a superconducting transition temperature of 71.4 K. in a P63/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 elements close to the junction of the different blocks (s-p; s-d; p-d; d-f) ie groups 2 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 Tc of 400 K around 300 GPa. could have a Tc 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 Tc = 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 Tc 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 .
See also Potassium nonahydridorhenate, stable at ordinary pressures
References