Uniaxial versus hydrostatic pressure-induced phase transitions in CaFe 2As 2 and BaFe 2As 2

We present uniaxial pressure structural relaxations for CaFe ₂As ₂ and BaFe ₂As ₂ within density functional theory and compare them with calculations under hydrostatic pressure as well as available experimental results. We find that CaFe ₂As ₂ 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, BaFe ₂As ₂ 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 experimental observations.