  A time-resolved synchrotron X-ray total scattering study sheds light on the
evolution of the different structural length scales involved during the
intercalation of the layered iron-selenide host by organic molecular donors,
aiming at the formation of the expanded lattice $Li_x(C_5H_5N)_yFe_{2-z}Se_2$
hybrid superconductor. The intercalates are found to crystallize in the
tetragonal $ThCr_2Si_2$-type structure at the average level, however, with an
enhanced interlayer iron-selenide spacing (d= 16.2 \r{A}) that accommodates the
heterocyclic molecular spacers. Quantitative atomic pair distribution function
(PDF) analysis at variable times, suggests distorted $FeSe_4$ tetrahedral local
environments that appear swollen with respect to those in the parent
$\beta$-FeSe. Simultaneously acquired, in-situ synchrotron X-ray powder
diffraction data disclose that secondary phases ($\alpha$-Fe and $Li_2Se$),
grow significantly when higher Li-concentration is used in the solvothermal
reaction or when the solution is aged. These observations are in line with the
strongly reducing character of the intercalation medium's solvated electrons
that mediate the defect chemistry of the expanded lattice superconductor. In
the latter, intralayer correlated local distortions indicate electron donating
aspects that reflect in somewhat enlarged Fe-Se bonds. They also reveal a
degree of relief of chemical pressure associated with a large distance between
Fe and Se sheets ('taller' anion height) and a stretched Fe-Fe square planar
topology. The elongation of the latter, derived from the in-situ PDF study,
speaks for a plausible increase in the Fe-site vacancy concentration. The
evolution of the local structural parameters suggests an optimum reaction
window where kinetically stabilized phases resemble the distortions of the
edge-sharing Fe-Se tetrahedra, required for high-$T_c$ in expanded lattice
iron-chalcogenides.
