Microscopic origin of pressure-induced phase transitions in iron pnictides

We have performed ab initio molecular dynamics calculations on the three materials (Ca, Sr, Ba)Fe$_2$As$_2$ under pressure. We find that the structural phase transition (orthorhombic to tetragonal symmetry) is always accompanied by a magnetic phase transition in all the compounds, while the nature of the transitions is different for the three systems. Our calculations explain the origin of the existence of a collapsed tetragonal phase in CaFe$_2$As$_2$ and its absence in BaFe$_2$As$_2$. We will also present a detailed analysis of pressure-induced superconductivity in BaFe$_2$As$_2$ based on neutron diffraction experiments, ARPES and bandstructure calculations and we will show the importance of structural distortions for the onset of superconductivity.