Local structure and phonon states mediated by intercalation-driven doping in superconducting Li1.0(C5H5N)yFe2−zSe2

Abstract

Intercalation of two-dimensional (2D) iron chalcogenides with molecular species requires disentangling electronic and structural contributions to understand the puzzling limit to superconducting transition temperature (Tc) at the frontier of long interlayer separations. Here, synchrotron x-ray absorption spectroscopy at the Se K edge sheds light on the impact of carrier doping on the local structure of the high-Tc (∼39K) Li1.0(C5H5N)yFe2−zSe2 phase. This material is derived by annealing the structurally related as-made derivative (Tc∼44K), with layers being primed apart by (alkali-molecule) guests. Metrics, such as a reduced filling of Se 4p orbitals and shorter Fe-Se bonds in the annealed phase, corroborate to a lower electron doping level with respect to the as-made one. Analysis of the metal-ligand thermal motion, based on the correlated Debye model, further relates the higher-Tc intercalates with the softening of the local Fe-Se bond. Beyond electronic effects, intercalation brings forth host-guest interactions that mediate the dynamics of the bulk crystal structure. For this, neutron time-of-flight spectroscopy on the annealed derivative corroborates the Se-Fe-Se layer being sensitive to chemical pressure effects imposed by the confined organic guests. This reflects in the phonon density of states, where harder low-energy transverse acoustic matrix phonons and molecular vibrations are witnessed, with respect to the pristine inorganic (β-FeSe) and organic (C5D5N) counterparts. On cooling through Tc, these excitations arrive without a collective magnetic-resonance mode—essential in unconventional, spin-mediated mechanisms—enquiring about deviations from optimal doping. The work highlights that when the Fe square planes are tuned far apart, carrier doping leveraged by intercalation plays a key role in the Tc parametrization.

Source-reported materials — not catalogue approval

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