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

Abstract

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.

Keywords:

• Ferroelectrics
• High pressure
• Phase diagram
• Phase transition
• X-ray diffraction
• Raman scattering

Acknowledgements

We acknowledge R. Ouillon, J.P. Pinan and P. Ranson for the Raman investigation of the R–O–T sequence at atmospheric pressure, and B. Canny for appreciable helps concerning the preparations of the experiments and comments on the data treatments. We thank also Ph. Baranek for helpful discussions and relevant results obtained from Hartree–Fock computations.