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High-pressure hydrides

58 sources, listed at the end · last revised 7 October 2026

High-pressure hydrides are hydrogen-rich compounds that become superconducting when compressed to pressures of the order of one megabar: 100 gigapascals (GPa), close to one million atmospheres. The highest transition temperatures reproduced in any material belong to this family: 203 kelvin (K; −70 °C) in the sulfur hydride H₃S at 155 GPa, about 1.5 million atmospheres, and about 250 K (−23 °C) in lanthanum decahydride, LaH₁₀, at about 170 GPa.^[1][2]

Neil Ashcroft proposed in 1968 that metallic hydrogen would superconduct at high temperature, and in 2004 that hydrogen-rich compounds would do so at pressures a laboratory can reach.^[8][9] Calculations gave the structure and approximate transition temperature of both record compounds before the measurements were published.^[10][11] The samples are less than a tenth of a millimeter across and are made and measured between the tips of two diamonds, and the record compounds have so far required megabar pressures.^[5][6][7]

Jorge Hirsch and Frank Marsiglio have disputed the evidence since 2020, and the two magnetic studies of H₃S and LaH₁₀ that they challenged have been corrected by their authors.^[12][13][14] Magnetic imaging by two other groups and a tunneling measurement of the superconducting energy gap have since added evidence independent of the disputed data.^[15][16][17] A 2025 preprint claims that the resistance of LaSc₂H₂₄ begins to fall, the onset of a transition, at 298 K (25 °C) and 260 GPa. It has not been reproduced, and a second group did not obtain the compound in seven attempts.^[3][4] Three other reports of hydride superconductivity, at 262 to 294 K and all from one group, were retracted between 2022 and 2024.^[18][19][20]

Why hydrogen

In a conventional superconductor, electrons pair by exchanging phonons, the vibrations of the crystal lattice (see Superconductivity). The transition temperature, T_c, rises with the frequency of those vibrations and with the strength of their coupling to the electrons. A vibrating atom behaves like a mass on a spring: the lighter the mass and the stiffer the spring, the higher the frequency. Hydrogen is the lightest atom, and in a dense metal its vibrations are fast and couple strongly to the electrons.^[1]

Ashcroft applied this reasoning to pure hydrogen,^[8] which is ordinarily an insulating gas of H₂ molecules. Calculations place its atomic metallic form at about 500 GPa (about 5 million atmospheres) and above, more than the pressure at the center of the Earth.^[21][5][22] In 2004 he argued that the same reasoning applies to alloys in which hydrogen is the dominant constituent, and that these should become metallic at pressures well below those needed for pure hydrogen, within the range of a diamond anvil cell.^[9] In such a compound the other atoms already hold the hydrogen at high density, an effect called precompression.^[7] Computer searches for stable crystal structures later named specific compounds and pressures: CaH₆ in 2012, H₃S in 2014 and LaH₁₀ in 2017.^[23][10][11] The methods are described in Predicting superconductors.

The diamond anvil cell

A diamond anvil cell presses a sample between the flattened tips of two opposed gem diamonds, with a thin metal gasket around the sample to contain it.^[24] Pressure is force divided by area, so the tips are small: 30 to 90 micrometers across in the hydride experiments cited, with samples 10 to 85 micrometers wide and 1 to 3 micrometers thick.^[5][3][6]

A superhydride, a hydride with far more hydrogen than the metal takes up at ordinary pressure, is made inside the cell. A flake of metal is loaded with hydrogen gas or a solid hydrogen source such as ammonia borane (NH₃BH₃), compressed, and heated by a laser to between 1,500 and 2,000 K so that metal and hydrogen react.^[5][27] X-ray diffraction at a synchrotron, an accelerator used as an intense X-ray source, locates the metal atoms only, and the hydrogen content is inferred from the volume of the crystal cell.^[3]

Measuring inside the cell

Electrical resistance is measured through four electrodes that reach the sample between the diamonds, two carrying a current and two sensing the voltage (a four-probe measurement).^[1][28]

A superconductor also expels a magnetic field from its interior, the Meissner effect. This second signature is sought because resistance can fall for other reasons (see Verifying a superconductor), but it is harder to measure: the sample has about one hundred-millionth of the mass of the cell around it.^[29] In one method a non-magnetic miniature cell 8.8 mm in diameter is placed inside a magnetometer based on a superconducting quantum interference device (SQUID).^[6][27] In another, small coils detect the change in magnetic susceptibility in an alternating field.^[30][27] A third uses nitrogen-vacancy centers, defects implanted in the tip of one diamond that act as optically read magnetic sensors and map the field across the sample with a resolution below one micrometer.^[15]

Sulfur hydride

Calculations had predicted that hydrogen sulfide, H₂S, would superconduct at about 80 K under pressure, and a calculation published in 2014 gave 191 to 204 K at 200 GPa for a compound richer in hydrogen, H₃S.^[1][10] A group led by Mikhail Eremets at the Max Planck Institute for Chemistry in Mainz compressed H₂S and saw its resistance fall to zero on cooling. The transition moved to lower temperature in a magnetic field and in the compound made with deuterium, the isotope of hydrogen with twice the mass (an isotope effect), and magnetic susceptibility measurements gave a T_c of 203 K (−70 °C), near 155 GPa.^[1] This exceeded the 164 K onset measured in a mercury cuprate at 31 GPa in 1994, until then the highest of any material.^[31][1]

Combined X-ray diffraction and resistance measurements showed that H₂S decomposes under pressure and that the superconducting phase is H₃S, with a cubic structure.^[32] A group at Jilin University detected the transition by alternating-current susceptibility, with a maximum T_c of 183 K at 149 GPa.^[30] Measurements on a Mainz sample by a team at Los Alamos National Laboratory and the US National High Magnetic Field Laboratory, and on a sample that a University of Bristol group made by a different route, gave transition temperatures near 200 K and upper critical fields that agree within experimental error.^[33][29]

Lanthanum hydride

Calculations published in 2017 predicted that LaH₁₀ would be stable above 200 GPa and superconduct at 274 to 286 K at 210 GPa.^[11] In its structure each lanthanum atom sits at the center of a cage of hydrogen atoms, an arrangement called a clathrate.^[2]

In August 2018 a group including Maddury Somayazulu and Russell Hemley reported a drop in resistance beginning near 260 K at 190 GPa, with drops beginning as high as 280 K in two further samples.^[28] The Mainz group then reported a transition at about 250 K (−23 °C) at about 170 GPa in which the resistance fell to zero, an isotope effect, and a T_c that falls in a magnetic field. T_c traced a dome against pressure, peaking at 250 to 252 K.^[2] A group in Beijing measured about 250 K at 165 GPa in 2020, and a group at Jilin University measured 248 K at 170 GPa in 2025, as a control in the preprint that reports LaSc₂H₂₄.^[34][3] The figures of 260 K and above come from resistance drops in one laboratory; about 250 K is the value other groups have reproduced.^[28][2][34]

Reproduced hydrides

A 2024 review by Eremets, whose group reported four of them, names seven hydrides whose superconductivity several independent groups have confirmed.^{[27][1][2][35]}

The two yttrium compounds were measured about 30 K below their predicted values, and YH₁₀, predicted to superconduct above 300 K, was not found at pressures up to 410 GPa and temperatures up to 2,250 K.^[35] CaH₆ was predicted in 2012 to superconduct at 220 to 235 K at 150 GPa, and two groups made it about a decade later.^[23][37][39]

Evidence and dispute

Lines of evidence

In each compound in the table the resistance falls to zero within the resolution of the measurement, and a magnetic field moves the transition to lower temperature; for YH₉ the field measurement was made on the deuterium compound.^{[1][2][35][36][37][38]} Replacing hydrogen with deuterium lowers T_c in H₃S, LaH₁₀, YH₆, YH₉ and the cerium hydrides, the isotope effect expected when lattice vibrations cause the pairing.^{[1][2][36][35][38]}

The Mainz group reported a susceptibility transition in H₃S in 2015, and in 2022 and 2023 two further effects in H₃S and LaH₁₀: screening, in which currents at the surface of the sample keep an applied field out, and trapped flux, a field that remains in the sample after the applied field is removed.^[1][6][40] Imaging with nitrogen-vacancy sensors by another collaboration showed local diamagnetism and trapped flux in CeH₉, in the same cell as the resistance drop, with superconducting regions uneven on the scale of micrometers.^[15] A further group extended the method to nearly 180 GPa and reported screening and the Meissner effect in lanthanum hydride at about 240 K and 155 GPa; an insufficiently annealed sample was uneven and had a lower T_c of about 220 K.^[16]

In 2025 a team including Eremets measured the superconducting energy gap, the energy needed to break an electron pair, by tunneling spectroscopy, in which electrons cross a thin barrier into the sample. The gap, written 2Δ, was about 60 millielectronvolts (meV) in H₃S and about 44 meV in the deuterium compound, with no electronic states inside it, and the authors state that the result supports pairing by lattice vibrations.^[17]

Criticism and replies

From 2020 Hirsch and Marsiglio argued in a series of papers that the hydride results are not superconductivity. An October 2020 preprint, later published as a comment in Nature, held that the resistive transition of carbonaceous sulfur hydride, whose report was later retracted, stayed too sharp in a magnetic field, in which the transition of a type II superconductor broadens, and that earlier hydride claims rested on similar or weaker evidence.^[12] They attributed the trapped-flux signal to ferromagnetism, the magnetism of materials such as iron, in the sample or its background.^[41]

The Mainz group replied in 2022 with a compilation of resistance, susceptibility and spectroscopic measurements, and stated that the objections could be refuted or explained.^[42] The group's 2022 paper on screening received an Author Correction in 2023, a comment on that correction by Hirsch in 2024, and on 6 October 2026 a second Author Correction, an Addendum and the authors' reply; its paper on trapped flux was corrected in 2025 (the corrections to the screening paper are described in Verifying a superconductor).^{[13][43][44][45][46][14]}

In a comment published in December 2024, fifteen condensed-matter physicists from outside the hydride field reviewed several of its papers and concluded that hydride superconductivity is real, citing the agreement between transition temperatures and critical fields measured by different groups. They asked that data be made public.^[29] Hirsch disputed their argument in a reply published in April 2025.^[47] The nitrogen-vacancy imaging and the tunneling gap were obtained by methods that do not use the disputed magnetization data.^[15][16][17]

Decompression and metastability

Each of the hydrides above is thermodynamically stable only within a range of pressure, outside which some other combination of phases has lower energy. LaSc₂H₂₄ is calculated to be stable between 167 and 300 GPa.^[48]

A phase can survive outside its range if an energy barrier prevents its atoms from rearranging. Such a phase is metastable; diamond at ambient pressure is the familiar example.

When LaH₁₀ is decompressed below about 135 GPa its cubic structure distorts, and T_c falls from about 243 K at 138 GPa to about 190 K at 120 GPa.^[5] The diffraction peaks of the LaSc₂H₂₄ samples split when the pressure was lowered below 194 GPa.^[3] A 2022 calculation for BaSiH₈, then not yet made and predicted to superconduct near 71 K, found that the energy barrier against loss of hydrogen molecules shrinks from 153 meV per atom at 100 GPa to 9 meV per atom at 25 GPa, and its authors put the limit of its survival on decompression near 30 GPa.^[49] A 2026 preprint reports that cubic BaSiH₈ was made at 18 to 31 GPa and recovered at ambient pressure, and that the barium silicon hydrides it studied were semiconductors or poor metals below 50 GPa.^[50]

Lowering the pressure

One route to lower pressure adds a second metal to make a ternary hydride. LaBeH₈, synthesized at 110 to 130 GPa, superconducts at up to 110 K at 80 GPa.^[51]

A second route is to search by calculation for hydrides that would superconduct with no applied pressure. One such prediction is Mg₂IrH₆, with calculated transition temperatures from about 65 to about 170 K depending on the method.^[52][53][54] It had not been made as of 2026. Syntheses produced the insulator Mg₂IrH₅ under every condition tested up to about 28 GPa,^[54] and above about 40 GPa the insulator Mg₂IrH₇, which reverts to Mg₂IrH₅ near 20 GPa on decompression.^[55] The related compound Mg₂RhH₆, made in 2026 above 30 GPa, superconducts at 24 K at 30 GPa and 29 K at 53 GPa, about half the predicted temperature, and reverts to Mg₂RhH₅ below about 30 GPa.^[56]

A 2025 study calculated the electron-phonon coupling of more than 20,000 metals at ambient pressure and found that compounds with higher predicted T_c are increasingly unstable. Its authors called conventional superconductivity at room temperature and ambient pressure "extremely unlikely".^[57]

The lanthanum scandium hydride claim

In 2024 a team led from Jilin University predicted a hexagonal compound, LaSc₂H₂₄, with a calculated T_c of up to 331 K at 250 GPa.^[48] On 29 September 2025 a group with overlapping authors posted a preprint reporting its synthesis from a lanthanum-scandium alloy and ammonia borane, laser-heated at 250 to 260 GPa. The preprint reports resistive onsets of 271 to 298 K between 195 and 266 GPa, the highest at 260 GPa, and zero resistance in two cells, with no measurement of magnetization or susceptibility and no deuterium experiment.^[3]

As of October 2026 the claim has not been reproduced, and the arXiv record of the preprint lists no journal publication.^[3] In a preprint posted in May 2026 and revised that August, Dmitrii Semenok and coauthors reported seven attempts to synthesize the compound at 250 to 280 GPa: no cell was shown to contain the hexagonal phase, and the four that gave resistance data showed no superconductivity between 245 and 300 K.^[4] In July 2026 the originating group published a calculation that assigns an unidentified second phase seen in some samples to La₂ScH₃₆, and predicts 261 to 282 K for a hydrogen-deficient form of it.^[58]

Retracted claims

Three reports of hydride superconductivity between 262 K and 294 K, all from one research group, were retracted: carbonaceous sulfur hydride at 288 K and 267 GPa (published 2020, retracted 26 September 2022), yttrium superhydride at 262 K (published 2021, retracted 13 June 2024), and nitrogen-doped lutetium hydride at 294 K and 1 GPa (published 2023, retracted 7 November 2023).^[18][19][20] The cases are described in Claims of room-temperature superconductivity.

See also

• Room-temperature superconductor
• Superconductivity
• Predicting superconductors
• Verifying a superconductor
• Claims of room-temperature superconductivity
• Superconducting materials
• History of superconductivity

References

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