Uniaxial versus hydrostatic pressure-induced phase transitions in CaFe2As2 and BaFe2As2

TL;DR

This study compares uniaxial and hydrostatic pressure effects on phase transitions in CaFe2As2 and BaFe2As2 using density functional theory, revealing high sensitivity to uniaxial stress and implications for superconductivity.

Contribution

It provides a detailed computational comparison of pressure-induced phase transitions under different stress conditions in two iron arsenide compounds, highlighting their sensitivity to uniaxial stress.

Findings

CaFe2As2 transitions directly from magnetic orthorhombic to nonmagnetic collapsed tetragonal phase.

BaFe2As2 exhibits two phase transitions, including an intermediate tetragonal phase.

Critical transition pressures are much lower under uniaxial pressure than hydrostatic pressure.

Abstract

We present uniaxial pressure structural relaxations for CaFe2As2 and BaFe2Ase2 within density functional theory and compare them with calculations under hydrostatic pressure conditions as well as available experimental results. We find that CaFe2As2 shows a unique phase transition from a magnetic orthorhombic phase to a nonmagnetic collapsed tetragonal phase for both pressure conditions and no indication of a tetragonal phase is observed at intermediate uniaxial pressures. In contrast, BaFe2As2 shows for both pressure conditions two phase transitions from a magnetic orthorhombic to a collapsed tetragonal phase through an intermediate nonmagnetic tetragonal phase. We find that the critical transition pressures under uniaxial conditions are much lower than those under hydrostatic conditions manifesting the high sensitivity of the systems to uniaxial stress. We discuss the origin of this sensitivity and its relation to superconductivity.