Hydrogen-rich compounds are extensively explored as candidates for a high-temperature superconductors. Currently, the measured critical temperature of $203$ K in hydrogen sulfide (H$_3$S) is among the highest over all-known superconductors. In presen
t paper, using the strong-coupling Eliashberg theory of superconductivity, we compared in detail the thermodynamic properties of two samples containing different hydrogen isotopes H$_3$S and D$_3$S at $150$ GPa. Our research indicates that it is possible to reproduce the measured values of critical temperature $203$ K and $147$ K for H$_3$S and D$_3$S by using a Coulomb pseudopotential of $0.123$ and $0.131$, respectively. However, we also discuss a scenario in which the isotope effect is independent of pressure and the Coulomb pseudopotential for D$_3$S is smaller than for H$_3$S. For both scenarios, the energy gap, specific heat, thermodynamic critical field and related dimensionless ratios are calculated and compared with other conventional superconductors. We shown that the existence of the strong-coupling and retardation effects in the systems analysed result in significant differences between values obtained within the framework of the Eliashberg formalism and the prediction of the Bardeen-Cooper-Schrieffer theory.
