High-pressure polymorphism of BaFe2Se3

TL;DR

This study investigates the high-pressure structural and superconducting properties of BaFe2Se3, revealing intrinsic superconductivity at 5 GPa and two structural phase transitions up to 20 GPa, with superconductivity suppressed at higher pressures.

Contribution

The paper provides the first detailed high-pressure structural and electrical analysis of BaFe2Se3, demonstrating intrinsic superconductivity and identifying two structural phase transitions.

Findings

Superconductivity observed at 5 GPa with Tc ~10 K.

Two structural transitions: Pnma to Cmcm above 3.5 GPa, and a first order transition at 16.6 GPa.

Superconductivity is suppressed beyond 20 GPa, with the material remaining insulating.

Abstract

BaFe2Se3 is a potential superconductor material exhibiting transition at 11 K and ambient pressure. Here we extended the structural and performed electrical resistivity measurements on this compound up to 51 GPa and 20 GPa, respectively, in order to distinguish if the superconductivity in this sample is intrinsic to the BaFe2Se3 phase or if it is originating from minor FeSe impurities that show a similar superconductive transition temperature. The electrical resistance measurements as a function of pressure show that at 5 GPa the superconducting transition is observed at around 10 K, similar to the one previously observed for this sample at ambient pressure. This indicates that the superconductivity in this sample is intrinsic to the BaFe2Se3 phase and not to FeSe with Tc > 20 K at these pressures. Further increase in pressure suppressed the superconductive signal and the sample remained in an insulating state up to the maximum achieved pressure of 20 GPa. Single-crystal and powder X-ray diffraction measurements revealed two structural transformations in BaFe2Se3: a second order transition above 3.5 GPa from Pnma (CsAg2I3-type structure) to Cmcm (CsCu2Cl3-type structure) and a first order transformation at 16.6 GPa. Here, {\gamma}-BaFe2Se3 transforms into {\delta}-BaFe2Se3 (Cmcm, CsCu2Cl3-type average structure) via a first order phase transition mechanism. This transitions is characterized by a significant shortening of the b lattice parameter of {\gamma}-BaFe2Se3 (17%) and accompanied by an anisotropic expansion in the orthogonal ac plane at the transition point.