Thermodynamic parameters of the LaH10 superconductor were an object of our interest. LaH10 is characterised by the highest experimentally observed value of the critical temperature: T a C = 215 K (pa = 150 GPa) and T b C = 260 K (p b = 190 GPa). It belongs to the group of superconductors with a strong electron-phonon coupling (λa ∼ 2.2 and λ b ∼ 2.8). We calculated thermodynamical parameters of this superconductor and found that the values of the order parameter, the thermodynamic critical field, and the specific heat differ significantly from the values predicted by the conventional BCS theory.Due to the specific structure of the Eliashberg function for the hydrogenated compounds, the qualitative analysis suggests that the superconductors of the La δ X 1−δ H10-type (LaXH-type) structure, where X ∈ {Sc, Y}, would exhibit significantly higher critical temperature than TC obtained for LaH10. In the case of LaScH we came to the following assessments: T a C ∈ 220, 267 K and T b C ∈ 263, 294 K, while the results for LaYH were: T a C ∈ 218, 247 K and T b C ∈ 261, 274 K.

The H5S2 and H2S compounds are the two candidates for the low-temperature phase of compressed sulfur-hydrogen system. We have shown that the value of Coulomb pseudopotential (μ*) for H5S2 ([TC]exp = 36 K and p = 112 GPa) is anomalously high. The numerical results give the limitation from below to μ* that is equal to 0.402 (μ* = 0.589), if we consider the first order vertex corrections to the electron-phonon interaction). Presented data mean that the properties of superconducting phase in the H5S2 compound can be understood within the classical framework of Eliashberg formalism only at the phenomenological level (μ* is the parameter of matching the theory to the experimental data). On the other hand, in the case of H2S it is not necessary to take high value of Coulomb pseudopotential to reproduce the experimental critical temperature relatively well (μ* = 0.15). In our opinion, H2S is mainly responsible for the observed superconductivity state in the sulfur-hydrogen system at low temperature.

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In the framework of strong-coupling formalism, we have calculated the thermodynamic parameters of superconducting state in the YB6 compound. The values of critical temperature (TC) are 9.5 K and 7.9 K for the Coulomb pseudopotential µ ⋆ = 0.1 and 0.2, respectively. In the paper, we have determined the low temperature values of order parameter (∆(0)), specific heat jump at the critical temperature (∆C(TC )), and thermodynamic critical field (HC(0)). The dimensionless thermodynamic ratios: R∆ = 2∆ (0) /kBTC , RC = ∆C (TC ) /C N (TC ), and RH = TCC N (TC) /H 2 C (0) are equal to: R∆ (µ ⋆ ) ∈ {4.48, 4.35}, RC (µ ⋆ ) ∈ {2.62, 2.55}, and RH (µ ⋆ ) ∈ {0.146, 0.157}. Due to the significant strong-coupling and retardation effects (kBTC /ω ln ∼ 0.1) those values highly deviate from the predictions of BCS theory.

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