Pressure‐stabilized hydrides are a new rapidly growing class of high‐temperature superconductors, which is believed to be described within the conventional phonon‐mediated mechanism of coupling. Here, the synthesis of one of the best‐known high‐TC superconductors—yttrium hexahydride Im3¯m‐YH6 is reported, which displays a superconducting transition at ≈224 K at 166 GPa. The extrapolated upper critical magnetic field Bc2(0) of YH6 is surprisingly high: 116–158 T, which is 2–2.5 times larger than the calculated value. A pronounced shift of TC in yttrium deuteride YD6 with the isotope coefficient 0.4 supports the phonon‐assisted superconductivity. Current–voltage measurements show that the critical current IC and its density JC may exceed 1.75 A and 3500 A mm−2 at 4 K, respectively, which is higher than that of the commercial superconductors, such as NbTi and YBCO. The results of superconducting density functional theory (SCDFT) and anharmonic calculations, together with anomalously high critical magnetic field, suggest notable departures of the superconducting properties from the conventional Migdal–Eliashberg and Bardeen–Cooper–Schrieffer theories, and presence of an additional mechanism of superconductivity.

Highlights• Superconductivity in fcc-ThH10 at 159-161 K at the pressure 174 Gigapascals • Very wide interval of stability of fcc-ThH10 from 85 to 185 GPa. • Upper critical magnetic field ThH10 ~45 Т. • Novel discovered superhydride hcp-ThH9 with TC of 146 K (170 GPa) and upper critical field ~38 Т • Newly discovered thorium hydrides: I4/mmm-ThH4 and Cmc21-ThH6 Abstract Here we report targeted high-pressure synthesis of two novel high-TC hydride superconductors, P63/mmc-ThH9 and 3 ̅ -ThH10, with the experimental critical temperatures (TC) of 146 K and 159-161 K and upper critical magnetic fields (μHC) 38 and 45 Tesla at pressures 170-175 Gigapascals, respectively. Superconductivity was evidenced by the observation of zero resistance and a decrease of TC under external magnetic field up to 16 Tesla. This is one of the highest critical temperatures that has been achieved experimentally in any compounds, along with such materials as LaH10, H3S and HgBa2CaxCu2O6+z. Our experiments show that fcc-ThH10 has stabilization pressure of 85 GPa, making this material unique among all known high-TC metal polyhydrides. Two recently predicted Th-H compounds, I4/mmm-ThH4 (> 86 GPa) and Cmc21-ThH6 (86-104 GPa), were also synthesized. Equations of state of obtained thorium polyhydrides were measured and found to perfectly agree with the theoretical calculations. New phases were examined theoretically and their electronic, phonon, and superconducting properties were calculated.Graphical Abstract

Distribution of superconducting properties among metal hydrides was investigated using stateof-the-art computational simulation techniques. We proposed a search rule for high-T C metalhydrogen systems based on analysis of electronic structure of atomic s, d, f-orbitals. Results of actinides and lanthanides study show that they form highly symmetric superhydrides XH 7-9 at relatively low pressures. However, actinides do not exhibit high-temperature superconductivity (except for Th-H system) and should not be considered as materials appropriate for experimental studies, as well as all d m -elements with m > 4 including metal hydrides of the precious elements. A refinement rule based on monotonic behavior of the maximum achievable critical temperature as a function of d+f electrons, maxT C (N d+f ), was proposed for already known materials. Using this rule, the reported T C values for the higher hydrides in K-H, Zr-H, Hf-H and Ti-H systems were corrected. The dependences of maxT C on the group number, period, pressure, and phase composition of hydrides were investigated. Developed model enables to make new targeted predictions relating to existence of new superconducting compounds. For Mg-H, Sr-H, Ba-H, Cs-H, Rb-H, we predict the existence of new high-T C phases XH n with n ≥ 10. Electron doping of H-sublattice by pressure-driven delocalization of d,f-electrons is suggested as the key factor for determining superconductive properties of polyhydrides. Graphical abstract:

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Superhydrides have complex hydrogenic sublattices and are important prototypes for studying metallic hydrogen and high-temperature superconductors. Encouraged by the results on LaH10, in consideration of the differences between La and Pr, Pr-H system is especially worth studying because of the magnetism and valence-band f-electrons in element Pr. Here we successfully synthesized praseodymium superhydrides (PrH9) in laser-heated diamond anvil cells. Synchrotron X-ray diffraction (XRD) analysis demonstrated the presence of previously predicted F4 ̅ 3m-PrH9 and unexpected P63/mmc-PrH9 phases. Moreover, Fm3 ̅ m-PrH3, P4/nmm-PrH3-δ and Fm3 ̅ m-PrH1+x were found below 52 GPa. F4 ̅ 3m-PrH9 and P63/mmc-PrH9 were stable above 100 GPa in experiment. Experimental studies of electrical resistance in the PrH9 sample showed the emergence of superconducting transition (Tc) below 9 K and a dependent Tc on applied magnetic field. Theoretical calculations indicate that magnetic order and electron-phonon interaction coexist in a very close range of pressures in the PrH9 sample which may contribute to its low superconducting temperature Tc. Our results highlight the intimate connections among hydrogenic sublattices, density of states, magnetism and superconductivity in Pr-based superhydrides.

New stable thorium decahydride 3 ̅ -ThH10, a record high-temperature superconductor with TC up to 241 K (-32 о С), critical field HC up to 71 T and superconducting gap Δ0 = 52 meV at 80-100 GPa was predicted by evolutionary algorithm USPEX. Another phase 2 1 / -ThH7 was found to be a high-temperature superconductor with TC ~ 65 K. Analysis of superconducting state was performed within Eliashberg formalism and the dependencies of HC(T), Δ(T), TC(P) together with jump in the specific heat at critical temperature were calculated. Several other new thorium hydrides were predicted to be stable under pressure including ThH3, Th3H10, ThH4, ThH6. Thorium (which has s 2 d 2 electronic configuration) forms high-TC polyhydrides similar to those formed by s 2 d 1 metals (Y-La-Ac). Thoriumis the next member in Mg-Ca-Sc-Y-La-Ac family of elements forming high-TC superconducting hydrides.

The stability of numerous unexpected actinium hydrides was predicted via the evolutionary algorithm USPEX. The electron-phonon interaction was investigated for the hydrogen-richest and most symmetric phases: R3̅ m-AcH, I4/ mmm-AcH, and P6̅ m2-AcH. Predicted structures of actinium hydrides are consistent with all previously studied Ac-H phases and demonstrate phonon-mediated high-temperature superconductivity with T in the range of 204-251 K for R3̅ m-AcH at 200 GPa and 199-241 K for P6̅ m2-AcH at 150 GPa, which was estimated by directly solving the Eliashberg equation. Actinium belongs to the series of d elements (Sc-Y-La-Ac) that form high- T superconducting (HTSC) hydrides. Combining this observation with previous predictions of p-HTSC hydrides (MgH and CaH), we propose that p and d metals with low-lying empty orbitals tend to form phonon-mediated HTSC metal polyhydrides.

Recently a big number of works devoted to search for new hydrides with record high-temperature superconductivity and at the same time the successful synthesis of potential high-TC superconducting FeH5 was reported. We present a systematic search for stable compounds in the Fe-H system using variable-composition version of the evolutionary algorithm USPEX. All known (FeH, FeH3, FeH5) and several new Fe3H5, Fe3H13 and FeH6 iron hydrides were found to be stable, resulting in a very complex phase diagram with rich structural relationships between phases. We calculate electronic properties of two potentially high-TC FeH5 and FeH6 phases in the pressure range from 150 to 300 GPa. Indeed, hydrogen-rich FeH5 and FeH6 phases were found to be superconducting within Bardeen-Cooper-Schrieffer theory, with TC values of up to 46 K.