Tetragonal to orthorhombic phase transition of GdFeAsO studied by single-crystal X-ray diffraction

► Single-crystals diffraction experiment on the phase transition of a REFeAsO compound. ► A geometrical model is developed to quantify metric distortion and twin formation. ► Structure models for high-temperature and low-temperature phase are refined. ► Influence of the structural distortion already perceptible before peak splitting.