  We have investigated experimentally how properties of NdFeAsO0.8F0.2
superconductor affected due to the substitution of the Ca2+/Nd3+ doping. Based
on the XRD data refinement, various structural parameters such as lattice
parameters, bond angles, bond length, and etc. were studied. We have determined
the upper limit of the calcium solubility in the NdFeAsO0.8F0.2 phase and it is
restricted to x=0.05. Also, we have found that the lattice parameters and the
cell volume decreased by increasing the calcium content. According to the XRD
data analysis, we have argued that these reductions are due to the variations
in the bond lengths and the bond angles of (O/F)-Nd-(O/F) and As-Fe-As i.e.
alpha, beta upon increasing the calcium dopant. So, we have expected that the
superconducting transition temperature (TC) will be sensitive to the calcium
doping values. Experimentally, the TC of our samples was reduced from 53 K (for
x = 0) to 48 K (for x = 0.01) and 27 K (for x=0.025) and disappeared for our
other sample. Then we have studied the dependence of TC and bond angles, bond
length, the pnictogen height, and the lattice parameter to examine the
available theories from an empirical point of view. The consistency of our
experimental results and the theoretical reports based on the spin- and the
orbital-fluctuation theories shows that these models play an important role in
the pairing mechanism of the iron-based superconductors.
