We conducted a joint experimental−theoretical investigation of the high-pressure chemistry of europium polyhydrides at pressures of 86−130 GPa. We discovered several novel magnetic Eu superhydrides stabilized by anharmonic effects: cubic EuH 9 , hexagonal EuH 9 , and an unexpected cubic (Pm3n) clathrate phase, Eu 8 H 46 . Monte Carlo simulations indicate that cubic EuH 9 has antiferromagnetic ordering with T N of up to 24 K, whereas hexagonal EuH 9 and Pm3n-Eu 8 H 46 possess ferromagnetic ordering with T C = 137 and 336 K, respectively. The electron−phonon interaction is weak in all studied europium hydrides, and their magnetic ordering excludes s-wave superconductivity, except, perhaps, for distorted pseudohexagonal EuH 9 . The equations of state predicted within the DFT+U approach (U − J were found within linear response theory) are in close agreement with the experimental data. This work shows the great influence of the atomic radius on symmetry-breaking distortions of the crystal structures of superhydrides and on their thermodynamic stability.
Lanthanum (La) is the first member of the rare-earth series of elements that has recently raised considerable interest due to its unique superhydride LaH 10+δ and its superconducting properties. Although several studies have found superconductivity and phase transitions in metallic La, there has been a lack of experimental evidence for the equation of state and superconductivity over one megabar pressure. Herein, we extend pressure range up to 163 GPa to explore the equation of state and superconductivity of La via electrical transport and X-ray diffraction measurements. Le Bail refinement of experimental XRD patterns indicated that above 130 GPa orthorhombically distorted fcc-La ( →Fmmm) phase emerges with the fitted parameters of 3 rd order Birch-Murnaghan equation of state: V 100 = 16.48 (7) Å 3 , B 100 = 208 (26) GPa and B 100 ' = 9 (1) in the range of 100-160 GPa. The superconductivity emerges in the newly distorted fcc-La with an onset critical temperature T c of 6.1 K at 100 GPa, decreases to 1.5 K at 130 GPa, then begins to grow and reach 3.3 K at 150 GPa. We have extrapolated the upper critical magnetic field μ 0 H c2 as 0.32 T (Gingburg-Landau) and 0.43 T (WHH formalism) at 140 GPa. Ab initio calculations confirm electron-phonon coupling mechanism with predicted T c (A-D) = 2.2 K (μ* = 0.195), dT c /dP = 0.11-0.13 K/GPa and μ 0 H c2 (0) = 0.4 T and coherence length ξ BCS = 28 nm at 140 GPa.
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.
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