  We report the temperature dependent x-ray powder diffraction of the
FeAs-based superconductors in the range between 300 K and 95 K. In the case of
NdOFeAs we have detected the structural phase transition from the tetragonal
phase, with P4/nmm space group, to the orthorhombic phase,with Cmma space
group, over a broad temperature range from 150 K to 120 K, centered at T0 137K.
This transition is reduced, by about 30K, by the internal chemical pressure
going from LaOFeAs to NdOFeAs. On the contrary the superconducting critical
temperature increases from 27K to 51 K going from LaOFeAs to NdOFeAs doped
samples. The FeAs layers in all undoped 1111 and 122 systems suffer a tensile
misfit strain. The tensile misfit strain is reduced in 1111 and in 122 samples
and at optimum doping the misfit strain is close to zero. This result shows
that the normal striped orthorhombic Cmma phase competes with the
superconducting tetragonal phase. In the orthorhombic clusters the charges can
move only along the stripes in the b direction and are localized by the
magnetic interaction.
