First Principles Study of CaFe2As2 "Collapse" Under Pressure

TL;DR

This study uses first principles calculations to analyze pressure-induced phase transitions in CaFe2As2, revealing an enthalpically driven transition from an antiferromagnetic orthorhombic phase to a non-magnetic collapsed tetragonal phase, consistent with experimental observations.

Contribution

It provides a detailed first-principles analysis of the pressure-induced phase transition in CaFe2As2, highlighting the enthalpic nature and electronic structure changes involved.

Findings

Transition from orthorhombic to tetragonal phase at 0.36 GPa

Pseudogaps observed in both phases

Enhanced As(p)-Fe(d) hybridization in orthorhombic phase

Abstract

We perform first principles calculations on CaFe2As2 under hydrostatic pressure. Our total energy calculations show that though the striped antiferromagnetic (AFM) orthorhombic (OR) phase is favored at P=0, a non-magnetic collapsed tetragonal (cT) phase with diminished c-parameter is favored for P > 0.36 GPa, in agreement with experiments. Rather than a mechanical instability, this is an enthalpically driven transition from the higher volume OR phase to the lower volume cT phase. Calculations of electronic density of states reveal pseudogaps in both OR and cT phases, though As(p) hybridization with Fe(d) is more pronounced in the OR phase. We provide an estimate for the inter-planar magnetic coupling. Phonon entropy considerations provide an interpretation of the finite temperature phase boundaries of the cT phase.