  Quantum transitions in Fe-based systems are believed to involve spin, charge
and nematic fluctuations. Complex structural phase diagram in these materials
often emphasizes importance of covalency in their exotic properties, which is
directly linked to the local structural network and barely understood. In order
to address this outstanding issue, we investigate the evolution of structural
parameters and their implication in unconventional superconductivity of 122
class of materials employing extended x-ray absorption fine structure studies.
The spectral functions near the Fe K- and As K-absorption edge of CaFe2As2 and
its superconducting composition, CaFe1.9Co0.1As2 (Tc = 12 K) exhibit evidence
of enhancement of Fe contribution with Co-substitution near the Fermi level.
As-Fe and Fe-Fe bondlengths derived from the experimental data exhibit
interesting changes with temperature across the magneto-structural transition.
Curiously, the evolution in Co-doped composition is similar to its parent
compound despite absence of magneto-structural transition. In addition, we
discover anomalous change of Ca-X (X = Fe, As) bondlengths with temperature in
the vicinity of magneto-structural transition and disorder appears to be less
important presumably due to screening by the charge reservoir layer. These
results reveal evidence of doping induced evolution to the proximity to
critical behavior presumably leading to superconductivity in the system.
