Vibrational dynamics and phase diagram of KNbO3 up to 30 GPa and from 20 to ∼500K

The phase diagram of KNbO3 was investigated by Raman scattering up to 30 GPa and from 10 to 500 K (750 K at 2 GPa). Macroscopic ferroelectricity vanishes on compression and the various ferroelectric solids, rhombohedral, orthorhombic and tetragonal (R-O-T), were found to exist in close domains of the pressure-temperature plane. Specifically the T solid vanishes below 200 K and the R and O solids exist, very likely, down to 0 K. Quantum suppression of the R solid likely occurs below 100 K. The transition to the cubic paraelectric phase is of the displacive type at low temperature whereas it is of the order-disorder type at high temperature. In the cubic solid, first-order Raman scattering, assigned to structural distortion due to dynamical disorder, was observed. The distortion is orthorhombic below 200 K and tetragonal above, which may stem from the rapid decrease with temperature of the Nb ion dwelling time in its allowed sites.