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

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Polyhydrides are a novel class of superconducting materials with extremely high critical parameters, which is very promising for sensor applications. On the other hand, a complete experimental study of the best so far known superconductor, lanthanum superhydride LaH10, encounters a serious complication because of the large upper critical magnetic field HC2(0), exceeding 120–160 T. It is found that partial replacement of La atoms by magnetic Nd atoms results in significant suppression of superconductivity in LaH10: each at% of Nd causes a decrease in TC by 10–11 K, helping to control the critical parameters of this compound. Strong pulsed magnetic fields up to 68 T are used to study the Hall effect, magnetoresistance, and the magnetic phase diagram of ternary metal polyhydrides for the first time. Surprisingly, (La,Nd)H10 demonstrates completely linear HC2(T) ∝ |T – TC|, which calls into question the applicability of the Werthamer–Helfand–Hohenberg model for polyhydrides. The suppression of superconductivity in LaH10 by magnetic Nd atoms and the robustness of TC with respect to nonmagnetic impurities (e.g., Y, Al, C) under Anderson's theorem gives new experimental evidence of the isotropic (s‐wave) character of conventional electron–phonon pairing in lanthanum decahydride.

Polyhydrides are a novel class of superconducting materials with extremely high critical parameters, which is very promising for applications. On the other hand, complete experimental study of the magnetic phase diagram for the best so far known superconductor, lanthanum decahydride LaH10, encounters a serious complication because of the large upper critical magnetic field HC2(0), exceeding 120–160 T. Partial replacement of La atoms by magnetic Nd atoms results in a decrease of the upper critical field, which makes it attainable for existing pulse magnets. We found that addition of neodymium leads to significant suppression of superconductivity in LaH10: each atomic % of Nd causes decrease in TC by 10–11 K. Using strong pulsed magnetic fields up to 68 T, we constructed the magnetic phase diagram of the ternary (La,Nd)H10 superhydride, which appears to be surprisingly linear with HC2 ∝ |T – TC|. The pronounced suppression of superconductivity in LaH10 by magnetic Nd atoms and the robustness of TC with respect to nonmagnetic impurities (e.g., Y, Al, C) under Anderson’s theorem indicate the isotropic (s‑wave) character of conventional electron-phonon pairing in the synthesized superhydrides.