PRESSURE-INDUCED STRONG MODE-COUPLING AND PHASE-TRANSITIONS IN KNBO3

Potassium niobate single-crystal samples have been studied by Raman sc attering up to 200 kbar. The effect of pressure on KNbO3 is extremely large and there exists a rich variety of pressure-induced changes, Mod e softening is found for some of the characteristic Raman bands of the orthorhombic phase. Intensity transfer and frequency repelling have b een observed in the 200 cm(-1) region as a result of strong coupling b etween two B-1(TO) modes, whose frequencies have been brought closer u nder pressure. There are three new crystalline phases and an amorphous phase in this pressure range, and two of the phase transitions are th ought to be displacive type. The samples do not transform back to the ambient-pressure phase on decompression.