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]
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]
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]
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] |
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]
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 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]
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 gradually as its temperature is lowered, even down to near absolute zero, a superconductor has a characteristic critical temperature below which the resistance drops abruptly to zero. An electric current through a loop of superconducting wire can persist indefinitely with no power source.
High-temperature superconductivity is superconductivity in materials with a critical temperature above 77 K, the boiling point of liquid nitrogen. They are only "high-temperature" relative to previously known superconductors, which function at colder temperatures, close to absolute zero. The "high temperatures" are still far below ambient, and therefore require cooling. The first breakthrough of high-temperature superconductor was discovered in 1986 by IBM researchers Georg Bednorz and K. Alex Müller. Although the critical temperature is around 35.1 K, this new type of superconductor was readily modified by Ching-Wu Chu to make the first high-temperature superconductor with critical temperature 93 K. Bednorz and Müller were awarded the Nobel Prize in Physics in 1987 "for their important break-through in the discovery of superconductivity in ceramic materials". Most high-Tc materials are type-II superconductors.
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.
Palladium hydride is palladium metal with hydrogen within its crystal lattice. Despite its name, it is not an ionic hydride but rather an alloy of palladium with metallic hydrogen that can be written PdHx. At room temperature, palladium hydrides may contain two crystalline phases, α and β. Pure α-phase exists at x < 0.017 while pure β-phase exists at x > 0.58; intermediate values of x correspond to α–β mixtures.
A room-temperature superconductor is a hypothetical material capable of displaying superconductivity above 0 °C, operating temperatures which are commonly encountered in everyday settings. As of 2023, the material with the highest accepted superconducting temperature was highly pressurized lanthanum decahydride, whose transition temperature is approximately 250 K (−23 °C) at 200 GPa.
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Covalent superconductors are superconducting materials where the atoms are linked by covalent bonds. The first such material was boron-doped synthetic diamond grown by the high-pressure high-temperature (HPHT) method. The discovery had no practical importance, but surprised most scientists as superconductivity had not been observed in covalent semiconductors, including diamond and silicon.
Iron(II) selenide refers to a number of inorganic compounds of ferrous iron and selenide (Se2−). The phase diagram of the system Fe–Se reveals the existence of several non-stoichiometric phases between ~49 at. % Se and ~53 at. % Fe, and temperatures up to ~450 °C. The low temperature stable phases are the tetragonal PbO-structure (P4/nmm) β-Fe1−xSe and α-Fe7Se8. The high temperature phase is the hexagonal, NiAs structure (P63/mmc) δ-Fe1−xSe. Iron(II) selenide occurs naturally as the NiAs-structure mineral achavalite.
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Artem R. Oganov is a Russian theoretical crystallographer, mineralogist, chemist, physicist, and materials scientist. He is known mostly for his works on computational materials discovery and crystal structure prediction, studies of matter at extreme conditions, including matter of planetary interiors.
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Mikhail Ivanovich Eremets was a Belarusian experimentalist in high pressure physics, chemistry and materials science. He was particularly known for his research on superconductivity, having discovered the highest critical temperature of 250 K (-23 °C) for superconductivity in lanthanum hydride under high pressures. Part of his research contains exotic manifestations of materials such as conductive hydrogen, polymeric nitrogen and transparent sodium.
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Lanthanum decahydride is a polyhydride or superhydride compound of lanthanum and hydrogen (LaH10) that has shown evidence of being a high-temperature superconductor. It was the first metal superhydride to be theoretically predicted, synthesized, and experimentally confirmed to superconduct at near room-temperatures. It has a superconducting transition temperature TC around 250 K (−23 °C; −10 °F) at a pressure of 150 gigapascals (22×10 6 psi), and its synthesis required pressures above approximately 160 gigapascals (23×10 6 psi).
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Carbonaceous sulfur hydride (CSH) is a potential superconductor that was announced in October 2020 by the lab of Ranga Dias at the University of Rochester, in a Nature paper that was later retracted. It was reported to have a superconducting transition temperature of 15 °C (59 °F) at a pressure of 267 gigapascals (GPa), which would have made it the highest-temperature superconductor discovered. The paper faced criticism due to its non-standard data analysis calling into question its conclusions, and in September 2022 it was retracted by Nature. In July 2023 a second paper by the authors was retracted from Physical Review Letters due to suspected data fabrication, and in September 2023 a third paper by the authors about N-doped lutetium hydride was retracted from Nature.
Ranga P. Dias is a researcher with a primary interest in condensed matter physics. He was an assistant professor in the departments of Mechanical Engineering and Physics and Astronomy at the University of Rochester (UR), and a scientist at the UR Laboratory for Laser Energetics. As of November 19, 2024, he was no longer employed at UR.
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