Intrinsic crystal phase separation in the antiferromagnetic superconductor Rb(y)Fe(2-x)Se2: a diffraction study

Area of Science:

• Condensed Matter Physics
• Materials Science
• Superconductivity

Background:

• Superconducting iron-based chalcogenides like Rb(y)Fe(2-x)Se(2) exhibit complex crystal and magnetic structures.
• Understanding these structures is crucial for elucidating their superconducting properties.

Purpose of the Study:

• To investigate the crystal and magnetic structures of Rb(y)Fe(2-x)Se(2) across a wide temperature range.
• To characterize the phase separation and its impact on the material's properties.

Main Methods:

• Single-crystal synchrotron x-ray diffraction.
• High-resolution neutron powder diffraction.
• Variable temperature studies (2-570 K).

Main Results:

• The ground state is phase-separated into two distinct crystallographic phases: a main phase with vacancy-ordered superstructure (I4/m) and AFM Fe spins, and a minority phase (8-10% mass fraction) with a disordered structure (I4/mmm).
• The minority phase has a smaller unit cell volume and distinct temperature dependence.
• The minority phase merges with the main phase above T(P) = 475 K, and annealing near T(P) reduces its fraction.

Conclusions:

• Rb(y)Fe(2-x)Se(2) exhibits intrinsic phase separation in its ground state.
• The interplay between crystal structure, phase separation, and magnetic ordering influences superconductivity.
• Thermal treatments can modify the phase fractions and potentially tune superconducting properties.