<i>Ab initio</i> study of pressure-driven phase transition in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi mathvariant="normal">FePS</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi mathvariant="normal">FePSe</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>

In spite of recent findings about the pressure-driven insulator-to-metal phase transition, and emerging superconductivity of ${\mathrm{FePS}}_{3}$ and ${\mathrm{FePSe}}_{3}$, the knowledge about their atomic structures is still vague. Here, we investigate the pressure-driven structural phase transitions of ${\mathrm{FePS}}_{3}$ and ${\mathrm{FePSe}}_{3}$ from 0 to 35 GPa by using ab initio calculations. We find that the ${\mathrm{FePS}}_{3}\ensuremath{-}C2/m$ B-I structure transforms to the ${\mathrm{FePS}}_{3}\ensuremath{-}C2/m$ B-II phase at about 5 GPa. Then above 17 GPa, the ${\mathrm{FePS}}_{3}\ensuremath{-}P\overline{3}1m$ B-III phase becomes energetically favored. For ${\mathrm{FePSe}}_{3}$, with increasing pressure, the ${\mathrm{FePSe}}_{3}\ensuremath{-}R\overline{3}$ T-I phase transforms to the B-II phase at around 6 GPa and further to the B-III phase at about 15 GPa. Our calculation results are consistent with experimentally observed high-pressure induced cell volume collapse, spin crossovers, and insulator-metal transition in ${\mathrm{FePS}}_{3}$ and ${\mathrm{FePSe}}_{3}$, which shed light on understanding the high-pressure physics and phase transitions of ${\mathrm{FePS}}_{3}$ and ${\mathrm{FePSe}}_{3}$.

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