Structural disorder in perovskite ferroelectric crystals as revealed by XAFS

The question whether ferroelectrics of the perovskite structure are driven by displacive or order-disorder mechanisms has been decided currently in favor of displacive based primarily on their dynamic behavior, particularly the existence of a soft mode both below and above T_c. However, various recent experiments have suggested that perovskite ferroelectrics and other ferroelectric crystals, are, in fact, disordered above T_c, contrary to the displacive mechanism. Recent developments in X-ray Absorption Fine Structure (XAFS) analysis have facilitated detailed investigation of the temperature dependence of the local structure providing, for the first time, a direct quantitative way to investigate the form of the local potential energy and the extent of disorder in both pure and mixed ferroelectrics as well as pure and mixed antiferrodistortive crystals. The surprising result is that in all the crystals studied so far, the low temperature local distortions remain large well above T_c. The macroscopic distortions vary by essentially disordering these local distortions and not by their decrease, indicating that the phase transition driving mechanism has a significant order-disorder component.