A theoretical study of the Pnma and Rm phases of Sb2S3, Bi2S3, and Sb2Se3

Abstract

We report a comparative theoretical study of the Pnma and Rm phases of Sb₂S₃, Bi₂S₃, and Sb₂Se₃ close to ambient pressure. Our enthalpy calculations at 0 K show that at ambient pressure the Rm (tetradymite-like) phase of Sb₂Se₃ is energetically more stable than the Pnma phase, contrary to what is observed for Sb₂S₃ and Bi₂S₃, and irrespective of the exchange–correlation functional employed in the calculations. The result for Sb₂Se₃ is in contradiction to experiments in which all three compounds are usually grown in the Pnma phase. This result is further confirmed by free-energy calculations taking into account the temperature dependence of unit-cell volumes and phonon frequencies. Lattice dynamics and elastic tensor calculations further show that both the Pnma and Rm phases of Sb₂Se₃ are dynamically and mechanically stable at zero applied pressure. Since these results suggest that the formation of the Rm phase of Sb₂Se₃ should be feasible under close to ambient conditions, we provide a theoretical crystal structure and simulated Raman and infrared spectra to help in its identification. We also discuss the results of the two published works that have claimed to have synthesized tetradymite-like Sb₂Se₃. Finally, the stability of the Rm phase across the three group-15 A₂X₃ sesquichalcogenides is analysed based on their van der Waals gap and X–X in-plane geometry.