Disorder-induced universality and scaling in hole-doped iron-based superconductors.

Iron-based superconductors exhibit various magnetic and electronic phases that are highly sensitive to structural and chemical modifications. Elucidating the origins of these phases remains a central challenge. Here, using neutron and x-ray diffraction, we uncover a universal phase diagram that identifies disorder as a hidden tuning parameter governing these phase transitions. By analyzing nine hole-doped phase diagrams, we observe the emergence of a double-Q tetragonal magnetic phase in proximity to ideal FeAs₄ tetrahedral configurations, thereby demonstrating a strong link between bond-angle stabilization and magnetic transitions. Beyond stabilizing the double-Q phase, atomic disorder also influences charge doping and magnetic anisotropy. We further observe similar scaling behavior of the transition temperatures of the double-Q and the more prevalent orthorhombic single-Q magnetic phases, evidencing a unified origin of structural and magnetic properties linked to itinerant nesting instability. Our findings establish a comprehensive basis for understanding how chemical disorder, charge doping, and structural features collectively shape the magnetic and superconducting properties of iron-based superconductors.