Local structure and phonon states mediated by intercalation-driven doping in superconducting L i 1.0 ( C 5 H 5 N ) y F e 2 − z S e 2

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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 (${T}_{\mathrm{c}}$) 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-${T}_{\mathrm{c}}$ ($\ensuremath{\sim}39\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) $\mathrm{L}{\mathrm{i}}_{1.0}{({\mathrm{C}}_{5}{\mathrm{H}}_{5}\mathrm{N})}_{y}\mathrm{F}{\mathrm{e}}_{2\ensuremath{-}z}\mathrm{S}{\mathrm{e}}_{2}$ phase. This material is derived by annealing the structurally related as-made derivative (${T}_{\mathrm{c}}\ensuremath{\sim}44\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), 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-${T}_{\mathrm{c}}$ 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 ($\ensuremath{\beta}$-FeSe) and organic (${\mathrm{C}}_{5}{\mathrm{D}}_{5}\mathrm{N}$) counterparts. On cooling through ${T}_{\mathrm{c}}$, 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 ${T}_{\mathrm{c}}$ parametrization.

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