Anomalous mode coupling and phase transition of KNbO3 under high pressure

Raman spectra of KNbO3 under high pressure have been analyzed on the basis of the two-mode coupling model. It has been found that the phase transitions from orthorhombic to tetragonal and from tetragonal to cubic occur at 7 and 8 GPa, respectively, accompanied by an abrupt jump of the mode width of one mode. The mode coupling occurs even in the cubic phase.