Local structural changes in KNbO3 under high pressure

The local structure of the perovskite KNbO₃ at 77 and 300 K under high pressure, up to 15.8 GPa, has been investigated using the x-ray-absorption fine-structure (XAFS) technique. It is found that the local-structure symmetry does not change even though the average macroscopic crystal symmetry changes several times over this same temperature and pressure range. The existence of different local and macroscopic structures means that the local distortions from the average structure are disordered. Other unexpected results obtained from the XAFS measurements are evidence for a decreasing Nb-Nb displacement correlation length and a destabilization of the oxygen octahedra with increasing pressure.