Microscopic origin of pressure-induced phase transitions in the iron pnictide superconductors A Fe2 As2: An ab initio molecular dynamics study

Using ab initio molecular dynamics we investigate the electronic and lattice structure of A Fe2 As2 (A=Ca,Sr,Ba) 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 CaFe2 As2 and its absence in BaFe2 As2. We argue that changes in the Fermi-surface nesting features dominate the phase transitions under pressure rather than spin frustration or a Kondo scenario. The consequences for superconductivity are discussed.