Synchrotron X-ray Diffraction Study of BaFe2As2 and CaFe2As2 at High Pressures up to 56 GPa: Ambient and Low-Temperatures Down to 33 K

TL;DR

This study uses synchrotron X-ray diffraction to investigate structural phase transitions in BaFe2As2 and CaFe2As2 under high pressures up to 56 GPa, revealing a collapsed tetragonal phase at specific pressures and temperatures.

Contribution

It provides detailed high-pressure structural data and insights into phase transition pressures and mechanisms in BaFe2As2 and CaFe2As2, highlighting the role of unit cell volume and tetrahedral distortions.

Findings

Collapsed tetragonal phase occurs at similar unit cell volumes in both compounds.

Transition pressures differ significantly between the two compounds at different temperatures.

FeAs4 tetrahedra are less compressible and more distorted in the collapsed phase.

Abstract

We report high pressure powder synchrotron x-ray diffraction studies on MFe2As2 (M=Ba, Ca) over a range of temperatures and pressures up to about 56 GPa using a membrane diamond anvil cell. A phase transition to a collapsed tetragonal phase is observed in both compounds upon compression. However, at 300 (33) K in the Ba-compound the transition occurs at 26 (29) GPa, which is a much higher pressure than 1.7 (0.3) GPa at 300 (40) K in the Ca-compound, due to its larger volume. It is important to note that the transition in both compounds occurs when they are compressed to almost the same value of the unit cell volume and attain similar ct/at ratios. We also show that the FeAs4 tetrahedra are much less compressible and more distorted in the collapsed tetragonal phase than their nearly regular shape in the ambient pressure phase. We present a detailed analysis of the pressure dependence of the structures as well as equation of states in these important BaFe2As2 and CaFe2As2 compounds.