Structural and magnetic properties of CaFe(2)As(2) and BaFe(2)As(2) from first-principles density functional theory

The structural and magnetic properties of XFe2As2 (X=Ca, Ba) under high pressure are investigated within first-principles density functional theory (DFT) in the nonmagnetic, stripe antiferromagnetic, and checkerboard antiferromagnetic orders. While for X=Ca, we find a unique transition at rather low pressure from the magnetic orthorhombic to the nonmagnetic tetragonal phase, recognized as a compressed-tetragonal phase, for X=Ba we observe two different low-temperature phase transitions as a function of pressure: an antiferromagnetic/nonmagnetic transition at about 14 GPa and a structural transition toward a collapsed tetragonal phase at much higher pressure. A careful comparison with available experimental measurements reveals that DFT-based calculations are able to reproduce with very high accuracy the structural properties of both compounds except for well-defined pressure ranges: we argue that such discrepancies are meaningful, might have different origins in the two compounds, and may hide valuable information that needs to be further investigated.