Diffraction above room temperature, on powders and crystals of Pb(Fe0.5Nb0.5)O-3, using X-rays and synchrotron radiation, led to a novel and consistent picture of the symmetry of the phases in this disordered perovskite. The compound is a monoclinic ferroelectric at 293 K, undergoes a structural transition to another ferroelectric, tetragonal state at about 355 K and finally transforms into a cubic form at approximately 376 K. (C) 1997 Elsevier Science Ltd. [References: 23]