High-pressure hydrogen sulfide by diffusion quantum Monte Carlo.

Area of Science:

• Condensed matter physics
• Materials science
• Computational chemistry

Background:

• Understanding the phase behavior of hydrogen sulfide (H2S) at high pressures is crucial for predicting its properties.
• Previous studies using density functional theory (DFT) have proposed various structures and phase transitions for H2S above 150 GPa.
• Experimental data, particularly from X-ray diffraction, provides benchmarks for theoretical predictions.

Purpose of the Study:

• To accurately determine the ground-state enthalpy-pressure phase diagram of H2S decomposition products at high pressures (above 150 GPa).
• To revise and refine existing phase diagrams based on more accurate computational methods.
• To identify the most stable structures of HS2 and H3S under extreme pressure conditions.

Main Methods:

• Accurate diffusion Monte Carlo (DMC) simulations were employed to calculate the energies of various proposed H2S structures.
• The enthalpy-pressure relationships were systematically analyzed to determine phase stability.
• Comparison with existing density functional theory (DFT) results and experimental data was performed.

Main Results:

• The C2/c HS2 structure is found to be stable up to 440 GPa, transitioning to the C2/m phase at higher pressures.
• The C2/m phase of HS is predicted to be more stable than the I41/amd phase across the 150-400 GPa range, contradicting some DFT predictions.
• The Im-3m phase is identified as the most probable candidate for H3S, aligning with recent experimental X-ray diffraction findings.

Conclusions:

• The study provides a revised and more accurate enthalpy-pressure phase diagram for high-pressure H2S.
• Diffusion Monte Carlo simulations offer a more reliable approach for predicting high-pressure phases compared to standard DFT methods in this system.
• The findings support experimental observations and advance the understanding of hydrogen-rich materials under extreme conditions.