| Summary: | We report an ab initio study of Sb<sub>2</sub>S<sub>3</sub>, Sb<sub>2</sub>Se<sub>3</sub>, and Bi<sub>2</sub>S<sub>3</sub> sesquichalcogenides at hydrostatic pressures of up to 60 GPa. We explore the possibility that the <i>C2/m</i>, <i>C2/c</i>, the disordered <i>Im-3m</i>, and the <i>I4/mmm</i> phases observed in sesquichalcogenides with heavier cations, viz. Bi<sub>2</sub>Se<sub>3</sub>, Bi<sub>2</sub>Te<sub>3</sub>, and Sb<sub>2</sub>Te<sub>3</sub>, could also be formed in Sb<sub>2</sub>S<sub>3</sub>, Sb<sub>2</sub>Se<sub>3</sub>, and Bi<sub>2</sub>S<sub>3</sub>, as suggested from recent experiments. Our calculations show that the <i>C2/c</i> phase is not energetically favorable in any of the three compounds, up to 60 GPa. The <i>C2/m</i> system is also unfavorable for Sb<sub>2</sub>S<sub>3</sub> and Bi<sub>2</sub>S<sub>3</sub>; however, it is energetically favorable with respect to the <i>Pnma</i> phase of Sb<sub>2</sub>Se<sub>3</sub> above 10 GPa. Finally, the <i>I4/mmm</i> and the disordered body-centered cubic-type <i>Im-3m</i> structures are competitive in energy and are energetically more stable than the <i>C2/m</i> phase at pressures beyond 30 GPa. The dynamical stabilities of the <i>Pnma</i>, <i>Im-3m</i>, <i>C2/m</i>, and <i>I4/mmm</i> structural phases at high pressures are discussed for the three compounds.
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