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

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→P3m1 for NaFeAs, P4/nmm→Cmcm→I4mm for LiFeP, and P4/nmm→P3m1 →I4mm→P6(3)/mmc for LiFeAs under high pressure. The high pressure orthorhombic Cmcm phase preserved the structural features of FeX(4)(X = As, P) tetrahedral layers present in the ambient-pressure P4/nmm structure. However, the FeX(4) tetrahedrons in the Cmcm phase were clearly distorted, leading to changes in the electronic behavior around the Fermi level. Under higher pressures, the FeX(4) layered structural features were no longer persistent and three-dimensional crystal structures were stabilized in other P3m1, I4mm, and P6(3)/mmc phases, which featured FeAs(5)/FeAs(6) hexahedron and octahedrons, FeX(5) tetragonal pyramids, and FeAs(6) 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 FeX(5) unit, which might be favorable for charge localization.