Polarization dependence of Born effective charge and dielectric constant in KNbO{sub 3}
- Department of Physics, College of William and Mary, Williamsburg, Virginia 23187-8795 (United States)
The Born effective charge Z{sup {asterisk}} and dielectric tensor {epsilon}{sub {infinity}} of KNbO{sub 3} are found to be very sensitive to the atomic geometry, changing by as much as 27{percent} between the paraelectric cubic and ferroelectric tetragonal and rhombohedral phases. Subtracting the bare ionic contribution reveals changes of the dynamic component of Z{sup {asterisk}} as large as 50{percent}, for atomic displacements that are typically only a few percent of the lattice constant. Z{sup {asterisk}}, {epsilon}{sub {infinity}}, and all phonon frequencies at the Brillouin zone center were calculated using the {ital ab} {ital initio} linearized augmented plane-wave linear response method with respect to the reference cubic, experimental tetragonal, and theoretically determined rhombohedral ground-state structures. The ground-state rhombohedral structure of KNbO{sub 3} was determined by minimizing the forces on the relaxed atoms. In contrast to the cubic structure, all zone-center phonon modes of the rhombohedral structure are stable, and their frequencies are in good agreement with experiment. In the tetragonal phase, one of the soft zone-center modes in the cubic phase is stablized. In view of the small atomic displacements involved in the ferroelectric transitions, it is evident that not only the soft-mode frequencies but also the Born effective charge and dielectric constants are very sensitive to the atomic geometry. {copyright} {ital 1996 The American Physical Society.}
- OSTI ID:
- 388298
- Journal Information:
- Physical Review, B: Condensed Matter, Vol. 54, Issue 16; Other Information: PBD: Oct 1996
- Country of Publication:
- United States
- Language:
- English
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