Refined geometry and frozen phonons in KNbO₃

Abstract Full-potential LMTO calculations for KNbO3 reported up to now provided a reasonable description of off-center equilibrium displacements, including transversal optical Γ phonons. However, when addressing more sensitive phonon properties and the behavior of the ferroelectric instability over the Brillouin zone, the need was realized to achieve a substantially higher level of accuracy. In order to arrive at ultimately accurate results available with the LMTO method in the local density approximation, the stability of full-potential LMTO predictions for off-center displacements in KNbO3, as depending on the choice of basis and expansion cutoffs, has been thoroughly investigated. With the calculation setup thus optimized, supercell frozen phonon calculations aimed at the study of the chain-structure instability over the Brillouin zone have been done, and the long-wavelength limit of the LO phonon is discussed. Key Words: KNbO3 LMTOfrozen phononstotal energyferroelectric instabilitydensity functional theory