CaFe2As2

CaFe2As2 is an iron-based compound that serves as a parent member of the 122 family of iron pnictide superconductors. It crystallizes in the tetragonal ThCr2Si2-type structure at room temperature, consisting of FeAs layers separated by calcium layers. In this structure, iron forms a square lattice coordinated by arsenic in tetrahedral geometry. Upon cooling, undoped CaFe2As2 undergoes a coupled structural and magnetic transition to an orthorhombic phase with stripe-like antiferromagnetic order near 170 K.

Under hydrostatic pressure or chemical substitution, the antiferromagnetic/orthorhombic phase is suppressed and superconductivity can emerge. Doping at the Fe or As sites (for example with Co, Ni or P substitution) or applying pressure can induce superconducting phases in CaFe2As2. At modest pressures, CaFe2As2 can transition to a collapsed tetragonal phase, characterized by a substantial collapse of the c-axis and the formation of As–As bonds between adjacent FeAs layers; this phase typically exhibits reduced magnetism and is not conducive to superconductivity.

CaFe2As2 is a canonical example for studying the interplay of structure, magnetism, and superconductivity in iron pnictides. It is commonly studied in single-crystal form, using techniques such as X-ray and neutron diffraction, electrical transport, and spectroscopic probes (e.g., ARPES) to characterize its phases. It is part of the broader AFe2As2 family (A = Ca, Sr, Ba, Eu) sharing the ThCr2Si2 structure.