  The tetragonal-to-orthorhombic structural phase transition (SPT) in LaFeAsO
(La-1111) and SmFeAsO (Sm-1111) single crystals measured by high resolution
x-ray diffraction is found to be sharp while the RFeAsO (R=La, Nd, Pr, Sm)
polycrystalline samples show a broad continuous SPT. Comparing the
polycrystalline and the single crystal 1111 samples, the critical exponents of
the SPT are found to be the same while the correlation length critical
exponents are found to be very different. These results imply that the lattice
fluctuations in 1111 systems change in samples with different surface to volume
ratio that is assigned to the relieve of the temperature dependent superlattice
misfit strain between active iron layers and the spacer layers in 1111 systems.
This phenomenon that is missing in the AFe2As2 (A=Ca, Sr, Ba) "122" systems,
with the same electronic structure but different for the thickness and the
elastic constant of the spacer layers, is related with the different maximum
superconducting transition temperature in the 1111 (55 K) versus 122 (35 K)
systems and implies the surface reconstruction in 1111 single crystals.
