Tetragonal to Orthorhombic Transition of GdFeAsO Studied by   Single-Crystal Synchrotron X-Ray Diffraction
  A study of the tetragonal to orthorhombic phase transition of GdFeAsO is presented. Planes of the reciprocal space were reconstructed form single-crystal synchrotron X-ray diffraction data. By cooling below the structural transition temperature splitting of the Bragg reflections was observed corresponding to four different twin domain orientations. A model was developed to quantify the distortion of the lattice from the position of the splitted reflections relative to each other. Constrained 2D-Cauchy fits of several splitted reflections provided positions of the reflections. The influence of the structural distortion was detectable already above the structural transition temperature hinting at fluctuations in the tetragonal phase. 
F. Nitsche, Th. Doert, M. Ruck
cond-mat.supr-con