RFeAsO

RFeAsO, where R is a rare-earth element, is a family of layered iron-based pnictide compounds. They crystallize in the tetragonal ZrCuSiAs-type structure with alternating FeAs conducting layers and RO blocking layers; the formula denotes one R, one FeAs, and one O per unit, stacked along the c-axis.

Discovery and superconductivity: The class rose to prominence after superconductivity was discovered in LaFeAsO1−xFx in 2008, with Tc around 26 K. Replacing O with F or creating oxygen vacancies provides electron doping, suppressing the native magnetic order of the Fe sublattice and producing superconductivity. With different R ions, Tc values up to about 50–55 K have been reported for some Sm-, Nd-, and Pr-based members; exact Tc depends on the rare-earth ion and the doping level.

Magnetism and structure: The parent RFeAsO compounds are not superconducting; Fe moments order antiferromagnetically in a spin-density-wave state and the structure undergoes a concomitant distortion. The rare-earth ions can order magnetically at lower temperatures, adding complexity to the phase diagram and interacting with superconductivity in some members.

Synthesis and properties: Synthesis typically involves high-temperature solid-state reactions from RO, Fe, and As precursors, with fluorine sources for doping. The FeAs layers host the essential electronic structure, featuring Fe 3d-derived bands and nesting between electron and hole pockets; the pairing mechanism in superconducting samples remains a topic of study, with many proposals favoring unconventional pairing.

Relation: RFeAsO belongs to the 1111 family of iron-based superconductors, alongside other families such as 122 and 111. Research continues into doping strategies, pressure effects, and the interplay between magnetism and superconductivity.