  We present a study of charge transfer in Na-intercalated FeOCl and
polyaniline-intercalated FeOCl using high-resolution x-ray absorption
spectroscopy and resonant x-ray emission spectroscopy at the Fe-K edge. By
comparing the experimental data with ab-initio simulations, we are able to
unambiguously distinguish the spectral changes which appear due to
intercalation into those of electronic origin and those of structural origin.
For both systems, we find that about 25% of the Fe sites are reduced to Fe2+
via charge transfer between FeOCl and the intercalate. This is about twice as
large as the Fe2+ fraction reported in studies using Mossbauer spectroscopy.
This discrepancy is ascribed to the fact that the charge transfer occurs on the
same time scale as the Mossbauer effect itself. Our result suggests that every
intercalated atom or molecule is involved in the charge-transfer process, thus
making this process a prerequisite for intercalation. The Fe2+ fraction is
found to increase with pressure for polyaniline-FeOCl, hinting at an
enhancement of the conductivity in the FeOCl intercalation compounds under
pressure.
