Ab initio prediction of pressure-induced structural phase transition of superconducting FeSe

External pressure driven phase transitions of FeSe are predicted using \textit{ab initio} calculations. The calculations reveal that $α$-FeSe takes transitions to NiAs-type, MnP-type, and CsCl-type FeSe. Transitions from NiAs-type to MnP-type and CsCl-type FeSe is also predicted. MnP-type FeSe is also found to be able to transform to CsCl-type FeSe, which is easier from $α$-FeSe than the transition to MnP-type FeSe, but comparable to the transition from NiAs-type FeSe. The calculated electronic structures show that all phases of FeSe are metallic, but the ionic interaction between Fe-Se bonds becomes stronger and the covalent interaction becomes weaker when the structural phase transition occurs from $α$-FeSe to the other phases of FeSe. The experimentally observed decrease in $T_{c}$ of superconducting $α$-FeSe at high pressure may be due to a structural/magnetic instability, which exists at high pressure. The results suggest us to increase the $T_{c}$ of $α$-FeSe if such phase transitions are frustrated by suitable methods.

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