Orbital-Selective Instabilities and Spin Fluctuations at the Verge of Superconductivity in Interlayer-Expanded Iron Selenide

Understanding electron correlation-driven instabilities and their coupling to structural phases is essential for deciphering multiorbital pairing in unconventional superconductors. We investigate Lix(C5H5N)yFe2Se2 (x ∼ 0.6; y ∼ 0.7–0.9), a tetragonal β-FeSe intercalate with a superconducting transition temperature (Tc = 39 K) closely tied to an expanded Fe-layer spacing (∼11.4 Å). High-resolution synchrotron X-ray diffraction and core-level absorption spectroscopy reveal subtle lattice distortions on cooling without a symmetry-breaking transition. Instead, the material exhibits negative thermal expansion (NTE) in the two-dimensional Fe network below TS ∼ 70 K, and stiffening of local Se–Fe–Se bond dynamics near Tc. The spatially incoherent rearrangement of FeSe4 tetrahedra and the site-local fluctuations, signal reduced electron correlations compared to those of parent β-FeSe (Tc = 8 K). Complementary X-ray emission spectroscopy, a fast local probe of Fe 3d valence states, detects persistent local Fe spin moments below TS, unlike quenching in related systems. These findings indicate that decoupling of Fe planes leads to an electronically driven lattice instability. The latter emerges as NTE induced from weak, orbital-selective localization of in-plane Fe 3d states rather than conventional transverse vibrations. Governed by Hund's coupling, this selectivity permits coexistence of local spin fluctuations with itinerant d-electrons─critical for enhancing Tc. These results suggest that intercalation-driven d-orbital differentiation moderates electron correlations, providing a pathway to optimize the superconductivity in low-dimensional quantum materials.