High-pressure phase diagram of hydrogen and deuterium sulfides from first principles: structural and vibrational properties including quantum and anharmonic effects

TL;DR

This study uses first-principles calculations to analyze the structural and vibrational properties of high-pressure sulfur hydrides, revealing quantum and anharmonic effects, isotope influences, and the impact of stress conditions on phase transitions relevant to superconductivity.

Contribution

It provides a detailed first-principles analysis of quantum and anharmonic effects on phase transitions in sulfur hydrides, including the influence of stress anisotropy and isotope substitution.

Findings

Quantum anharmonic effects significantly lower the transition pressure.

Isotope substitution increases the transition pressure for D3S compared to H3S.

Anharmonic phonon spectra show large broadening but well-defined quasiparticles at zone center.

Abstract

We study the structural and vibrational properties of the high-temperature superconducting sulfur trihydride and trideuteride in the high-pressure $Im\bar{3}m$ and $R3m$ phases by first-principles density-functional-theory calculations. On lowering pressure, the rhombohedral transition $Im\bar{3}m \rightarrow R3m$ is expected, with hydrogen bond desymmetrization and occurrence of trigonal lattice distortion. In hydrostatic conditions we find that, contrary to what suggested in some recent experiments, if the rhombohedral distortion exists it affects mainly the hydrogen-bonds, whereas the resulting cell distortion is minimal. We estimate that the occurrence of a stress anisotropy of approximately $10\%$ could explain this discrepancy. Assuming hydrostatic conditions, we calculate the critical pressure at which the rhombohedral transition occurs. Quantum and anharmonic effects, which are relevant in this system, are included at nonperturbative level with the stochastic self-consistent harmonic approximation (SSCHA). Within this approach, we determine the transition pressure by calculating the free energy Hessian. We find that quantum anharmonic effects are responsible for a strong reduction of the critical pressure with respect to the one obtained with the classical harmonic approach. Moreover, we observe a prominent isotope effect, as we estimate higher pressure transition for D${}_3$S than for H${}_3$S. Finally, within SSCHA we calculate the anharmonic phonon spectral functions in the $Im\bar{3}m$ phase. The strong anharmonicity of the system is confirmed by the occurrence of very large anharmonic broadenings leading to complex non-Lorentzian line shapes. However, for the vibrational spectra at zone center, accessible e.g. by infrared spectroscopy, the broadenings are very small (linewidth at most around 2~meV) and anharmonic phonon quasiparticles are well defined.