  Direct quantitative correlations between the orbital order and
orthorhombicity is achieved in a number of Fe-based superconductors of 122
family. The former (orbital order) is calculated from first principles
simulations using experimentally determined doping and temperature dependent
structural parameters while the latter (the orthorhombicity) is taken from
already established experimental studies; when normalized, both the above
quantities quantitatively corresponds to each other in terms of their doping as
well as temperature variations. This proves that the structural transition in
Fe-based materials is electronic in nature due to orbital ordering. An
universal correlations among various structural parameters and electronic
structure are also obtained. Most remarkable among them is the mapping of two
Fe\--Fe distances in the low temperature orthorhombic phase, with the band
energies E$_{d_{xz}}$, E$_{d_{yz}}$ of Fe at the high symmetry points of the
Brillouin zone. The fractional co-ordinate $z_{As}$ of $As$ which essentially
determines anion height is inversely (directly) proportional to Fe-As bond
distances (with exceptions of K doped BaFe$_2$As$_2$) for hole (electron) doped
materials as a function of doping. On the other hand, Fe-As bond-distance is
found to be inversely (directly) proportional to the density of states at the
Fermi level for hole (electron) doped systems. Implications of these results to
current issues of Fe based superconductivity are discussed.
