KNbO3 under high pressure

The behaviors of ferroelectric KNbO3 under pressure were studied by in situ dielectric constant measurements in a cubic anvil press up to 11 GPa at lo w temperatures and by x-ray diffraction in a diamond anvil cell using synch rotron radiation and an imaging plate up to 65 GPa at room temperature. Bas ed on the dielectrical changes, it is suggested that the pressure-induced t ransitions from the ferroelectric orthorhombic phase to the ferroelectric t etragonal occurs at 8.5 GPa, which in turn, transforms to the paraelectric cubic phase at 11 GPa. All three phases are stable at ambient pressure at r oom temperature. A new structural transition was found at around 40 GPa. Th e high pressure phase appears to adopt the GdFeO3-type structure (space gro up: Pnma), which is one of the distorted perovskite structures. The sequenc e of phase transition in KNbO3 is therefore such that orthorhombic --> tetr agonal --> cubic --> orthorhombic (GdFeO3-type) with increasing pressure at room temperature.