PASS‐Derived O/S Donor Field‐Directed Competitive–Cooperative Fe/Co Phase Evolution for Cross‐Band Microwave Absorption

Fe/Co bimetallic regulation enables high-performance microwave absorbers, yet the role of intrinsic coordination disparity in precursor conversion, nonequilibrium phase evolution, and cross-band loss redistribution remains unclear. We design a poly(arylene sulfide sulfone) (PASS)-predefined O/S heterogeneous donor-field strategy to direct competitive-cooperative nonequilibrium phase evolution in Fe/Co systems. Covalently embedded sulfone-O and thioether-S sites, with PASS's rigid backbone, create a molecularly preorganized donor field that amplifies non-equivalent Fe/Co coordination, promoting competitive bonding, differentiated phase pathways, and electronic-state reconstruction during confined pyrolysis. Fe/Co regulation thus reshapes local phases, O/S chemical environments, and interfacial charge states via Fe─O/Co─S coordination preferences, generating multiphase heterostructures, amorphous/crystalline interfaces, defects, and lattice strain while balancing dielectric polarization, conductive transport, magnetic response, and impedance matching. The resulting CFC absorber achieves cross-band absorption in low-frequency 5G and high-frequency radar bands. CFC-4 exhibits -73.83 dB at 6.23 GHz (3.20 mm) and -76.59 dB at 16.98 GHz (1.31 mm), with a maximum EAB of 5.03 GHz. CFC-4∼6 (1-5 mm) show >90% cumulative effective absorption over 2-18 GHz. Finite-element simulations reveal a maximum RCS reduction of 42.5 dB on PEC. This work establishes O/S donor-field engineering as a viable route to amplify Fe/Co coordination differentiation for efficient cross-band microwave absorption.