Compressed tetragonal phase in XFe2As2 (X=Na, K, Rb, Cs) and in the alloy Na0.5 K0.5Fe2As2

Motivated by the recent discovery of a high-temperature superconducting phase in KFe2As2 at 16 GPa accompanied by an isostructural phase transition from the tetragonal to the compressed tetragonal phase, we extend the study of pressure effects to the whole XFe2As2 family (X=Na, K, Rb, Cs). We demonstrate that the ionic radius of the X atom determines the transition to the compressed phase which induces relevant changes in electronic properties and the Fermi surface which can enhance the superconducting pairing. Based on these results and in analogy with KFe2As2, we theoretically propose Na0.5K0.5Fe2As2 as a possible new superconductor material in its compressed tetragonal phase which we predict to happen above 6 GPa.