High pressure structures of “111” type iron-based superconductors predicted from first-principles

Abstract

The high-pressure crystal structures of the “111” type iron-based superconductors: NaFeAs, LiFeP and LiFeAs have been systematically explored by using particle-swarm structural searches. It was found that though these iron-based superconductors are chemically similar, they adopted distinct structural phase transitions: P4/nmm → Cmcm → Pm1 for NaFeAs, P4/nmm → Cmcm → I4mm for LiFeP, and P4/nmm → Pm1 → I4mm → P6₃/mmc for LiFeAs under high pressure. The high pressure orthorhombic Cmcm phase preserved the structural features of FeX₄(X = As, P) tetrahedral layers present in the ambient-pressure P4/nmm structure. However, the FeX₄ tetrahedrons in the Cmcm phase were clearly distorted, leading to changes in the electronic behavior around the Fermi level. Under higher pressures, the FeX₄ layered structural features were no longer persistent and three-dimensional crystal structures were stabilized in other Pm1, I4mm, and P6₃/mmc phases, which featured FeAs₅/FeAs₆ hexahedron and octahedrons, FeX₅ tetragonal pyramids, and FeAs₆ octahedrons, respectively. Analysis of the electronic density of states suggests that most of the high pressure phases are metallic except for the tetragonal I4mm phase, which possesses a narrow band gap. This semiconducting state might relate to the tetragonal pyramid structure formed by FeX₅ unit, which might be favorable for charge localization.