Structural, magnetic and superconducting phase transitions in CaFe2As2 under ambient and applied pressure

TL;DR

This study investigates the phase transitions of CaFe2As2 under ambient and applied pressure, revealing how pressure suppresses magnetic order and induces a collapsed tetragonal phase, with the low temperature state strongly influenced by the pressure medium.

Contribution

It provides detailed insights into pressure-induced phase transitions in CaFe2As2 and highlights the significant effects of pressure medium conditions on the material's low temperature phases.

Findings

Pressure suppresses the orthorhombic/antiferromagnetic phase.

Collapsed tetragonal phase appears above 0.35 GPa.

Pressure medium affects the low temperature phase stability.

Abstract

At ambient pressure CaFe2As2 has been found to undergo a first order phase transition from a high temperature, tetragonal phase to a low temperature orthorhombic / antiferromagnetic phase upon cooling through T ~ 170 K. With the application of pressure this phase transition is rapidly suppressed and by ~ 0.35 GPa it is replaced by a first order phase transition to a low temperature collapsed tetragonal, non-magnetic phase. Further application of pressure leads to an increase of the tetragonal to collapsed tetragonal phase transition temperature, with it crossing room temperature by ~ 1.7 GPa. Given the exceptionally large and anisotropic change in unit cell dimensions associated with the collapsed tetragonal phase, the state of the pressure medium (liquid or solid) at the transition temperature has profound effects on the low temperature state of the sample. For He-gas cells the pressure is as close to hydrostatic as possible and the transitions are sharp and the sample appears to be single phase at low temperatures. For liquid media cells at temperatures below media freezing, the CaFe2As2 transforms when it is encased by a frozen media and enters into a low temperature multi-crystallographic-phase state, leading to what appears to be a strain stabilized superconducting state at low temperatures.