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-absor ption fine-structure (XAFS) technique. It is found that the local-stru cture symmetry does not change even though the average macroscopic cry stal symmetry changes several times over this same temperature and pre ssure range. The existence of different local and macroscopic structur es means that the local distortions from the average structure are dis ordered. Other unexpected results obtained from the XAFS measurements are evidence for a decreasing Nb-Nb displacement correlation length an d a destabilization of the oxygen octahedra with increasing pressure. [S0163-1829(97)08241-6].