Fig. 1. Clathrate structures of typical metal superhydrides (polyhydrides).²¹ Maximum T_c is predicted for YH₁₀ and is slightly above room temperature at 250 GPa. [Reprinted Figs. 2 and 4 with permission from Peng et al., Phys. Rev. Lett. 119(10), 107001 (2017). Copyright (2017) The American Physical Society.]

Fig. 2. Predicted superconductivity in LaH₁₀ and YH₁₀ superhydrides.²² Blue and red curves and symbols correspond to the indicated values of the Morel–Anderson pseudopotential, μ*.

Fig. 3. X-ray diffraction of LaH₁₀ sample after laser heating at 175 GPa and 169 GPa.²⁴ Right panel: Volume per formula unit compared with theory and sum of the volumes of pure La and five H₂ units. [Reproduced with permission from Geballe et al., Angew. Chem., Int. Ed. 57, 688–692 (2018). Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA.]

Fig. 4. Resistivity studies of superconductivity in LaH₁₀.²⁵ Left panel: Typical arrangement of electrical contacts and x-ray transmission scan through the sample and contact area. Right panel: Resistivity signatures of superconductivity in LaH₁₀. [Reprinted Figs. S1 and 2 with permission from Somayazulu et al., Phys. Rev. Lett. 122(2), 027001 (2019). Copyright 2019 The American Physical Society.]

Fig. 5. Observed volumes per formula unit before and after the conversion from La to LaH_10±x for several samples probed by resistivity technique.²⁵ The predicted P–V curves for the assemblages La + 5H2 and La + 4H2 are plotted as the dashed and dash-dot lines (respectively). Samples which do not show evidence of the conductivity drop above 80 K lie below the La + 4H₂ line. Right panel shows the resistivity of one of the samples under varying pressure during thermal cycling (supplementary material in Ref. 25). [Reprinted Figs. S3 and S4 with permission from Somayazulu et al., Phys. Rev. Lett. 122(2), 027001 (2019). Copyright 2019 The American Physical Society.]

Fig. 6. Transition to a lower symmetry phase (R-3m) in LaH₁₀ on decompression from ∼170 GPa.²⁴ [Reproduced with permission from Geballe et al., Angew. Chem., Int. Ed. 57, 688–692 (2018). Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA.]

Fig. 7. Left panel: Schematic representation of the background subtraction principle in magnetic susceptibility measurements: 1. primary coil; 2. secondary compensating coil; 3. secondary signal coil. Further experimental details can be found in Ref. 38. “Removal” of the sample from the signal coil by applying an external magnetic field over the critical value produces measurable changes in the total output signal. On the right we show a typical signal of a cuprate superconductor compared with a model calculation for type II superconductors—see Ref. 38 for details of the model calculation.

Fig. 8. Left panel shows the magnetic response signals from the LaH₁₀ sample in a DAC. The superimposed signals at several pressures are shown by dots: blue—164 GPa, green—169 GPa, red—180 GPa; the background is shown as a red dashed line (approximated by 3rd degree polynomial). Right panel: Signal after background subtraction, shifted in vertical direction (from bottom to top: blue—164 GPa, green—169 GPa, red—180 GPa).

Fig. 9. Calibration curves for two setups operating at two different excitation frequencies.³⁵ The samples used for calibrating the amplitude of the response are various high-T_c cuprates, Nb, and MgB₂. The practical sensitivity limit for the 160 KHz setup is around 10⁻¹⁰ – 2 × 10⁻¹⁰ cm³, which is more than one order of magnitude lower in comparison with the typical sensitivity limit (10⁻⁸ e.m.u.) of an MPMS system from Quantum Design.

Fig. 10. Summary of the available experimental and theoretical data of superconductivity in LaH₁₀. Sources for the data: Somayazulu et al.,²⁵ Drozdov I et al.,³² Drozdov II et al.,²⁶ Liu et al.,²² Kruglov et al.,⁴⁰ and Peng et al.²¹

Fig. 11. Troyan et al.⁴¹ (a) XRD pattern of M3 sample at 172 GPa recorded at λ = 0.2952 Å; (b) Le Bail refinements for Im3¯m-YH6 and I4/mmm-YH4. Red circles are experimental data; black line is the fit; green line shows residues.

Fig. 12. Troyan et al.⁴¹ Superconducting transitions in Im3¯m-YH6: (a) Dependence of electrical resistance on temperature. Inset: the resistance drops to zero after cooling below ТC; (b) jump in R(T) dependence of resistance (nine times increase) on temperature for the second sample.

Fig. 13. Semenok et al.⁴⁴ Observation of superconductivity in (a) ThH₁₀ and (b) ThH₉. The temperature dependence of the resistance (R) of thorium superhydride was determined in a sample synthesized from Th+NH₃BH₃. The resistance was measured using four electrodes deposited on a diamond anvil with the sample placed on top of the electrodes [Fig. 15(a), inset] with an excitation current of 100 µA. The resistance near the zero point is shown on a smaller scale in the insets; (c) dependence of the resistance on temperature under an external magnetic field at 170 GPa; (d) dependence of the critical temperature (T_c) of ThH₁₀ and ThH₉ on the magnetic field.

Fig. 14. High-T_c superconducting superhydrides,⁴⁷ which is very similar to the highest T_c in elements under high pressure conditions (see e.g., http://www.hpr.stec.es.osaka-u.ac.jp/e-super) [The figure is reprinted with permission from supplementary material in Semenok et al., J. Phys. Chem. Lett. 9(8), 1920–1926 (2018). Copyright 2018 American Chemical Society.].

Fig. 15. Comparison of atomic volumes per hydrogen atom in high-T_c and nonsuperconducting hydrides. The equation of state for hypothetical atomic hydrogen (green dashed line) is taken from Pepin et al.,⁴⁹ as well as atomic H volumes for FeH₃ and FeH₅. The H₂ volume is from Loubeyre et al.⁵⁰ LaH₁₀ and AlH₃ data are taken from Geballe et al.²⁴ ThH₁₀ is taken from Semenok et al.,⁴⁴ YH₆ from Troyan et al.⁴¹ H₃S data are given as calculated in Ref. 51.