  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.
