Complex biphase nature of the superconducting dome of the FeSe phase diagram

TL;DR

This study reveals a complex biphase structure in FeSe under varying pressure, showing how structural changes influence superconductivity, with a detailed phase diagram and unambiguous high-pressure crystal structure determination.

Contribution

It provides the first detailed phase diagram linking FeSe's structural phases with its superconducting properties and clarifies the high-pressure crystal structure of FeSe.

Findings

Superconductivity is enhanced below 6 GPa with 2D layered structure.

Above 6 GPa, a transition to a 3D orthorhombic phase suppresses superconductivity.

The high-pressure phase has a MnP-type structure, confirmed unambiguously.

Abstract

Single crystal synchrotron X-ray diffraction as a function of temperature and pressure has revealed a complex biphase mixture in superconducting FeSe. Based on our experimental results we construct a phase diagram where structural behavior and superconducting properties of FeSe are found to be correlated. We show that below 6 GPa, where pressure promotes the superconducting critical temperature, the FeSe structure is composed of 2D layers of edge-shared FeSe4 tetrahedra, while above 6 GPa the superconductivity is strongly suppressed on formation of a new orthorhombic polymorph characterized by a 3D network of face sharing FeSe6 octahedra. Therefore changes in topology and connectivity of the FeSe structure are found to be detrimental for superconductivity to exist. This previously controversial crystal structure of the high pressure polymorph of FeSe was also unambiguously determined. High pressure FeSe adopts an orthorhombic MnP-type structure (Pnma) which corresponds to a slightly distorted hexagonal NiAs-type arrangement (P63/mmc). The structural transformation from the low- to high-pressure FeSe polymorph is first order in nature and is manifested as antiparallel displacements within the Fe and Se sublattices.