Polarization properties of KNbO [formula omitted]: comparison between Hartree–Fock and density-functional calculations

Abstract

We perform first-principles calculations on the ferroelectric perovskite KNbO 3 , using both Hartree–Fock (HF) and density functional theory in the local density approximation (LDA). We compute longitudinal and transverse dynamical charges, electronic dielectric constant, and the spontaneous polarization. Longitudinal dynamical charges are computed with a supercell technique, while transverse dynamical charges are computed through the Berry phase. Combining the two types of dynamical charges, the electronic dielectric constant is determined. The spontaneous polarization is also computed as a Berry phase. Band–by–band decomposition of the dynamical charges is performed and analyzed. The differences between the HF and LDA results are pointed out and their origin is scrutinized. The results show that in this material, having an intermediate ionic/covalent character, HF underestimates the covalence mechanism, while LDA overestimates it. Both methods provide results of comparable accuracy, but on opposite sides of the experimental values.