Consistent Atomic Geometries and Electronic Structure of Five Phases of Potassium Niobate from Density-Functional Theory

Abstract

We perform a comprehensive theoretical study of the structural and electronic properties of potassium niobate (KNbO3) in the cubic, tetragonal, orthorhombic, monoclinic, and rhombohedral phase, based on density-functional theory. The influence of different parametrizations of the exchange-correlation functional on the investigated properties is analyzed in detail, and the results are compared to available experimental data. We argue that the PBEsol and AM05 generalized gradient approximations as well as the RTPSS meta-generalized gradient approximation yield consistently accurate structural data for both the external and internal degrees of freedom and are overall superior to the local-density approximation or other conventional generalized gradient approximations for the structural characterization ofKNbO3. Band-structure calculations using a HSE-type hybrid functional further indicate significant near degeneracies of band-edge states in all phases which are expected to be relevant for the optical response of the material.

Electronic and Optical Properties of Sodium Niobate: A Density Functional Theory Study

“…is is in line with similar investigations for the other end member KNbO 3 [21] of the solid solution sodium potassium niobate (Na,K)NbO 3 . Based on the PBEsol ground state geometries, the electronic band structures and the optical dielectric functions have been calculated and discussed with respect to available experimental data, with the PBEsol functional performing best.…”

“…In addition, we also perform benchmark calculations for the simple cubic PE perovskite phase (Pm3m) utilising the hybrid functional PBE0 [19], where a quarter of the exchange potential is replaced by Hartree-Fock exact-exchange to better account for electronic correlation effects [20]. In accordance with similar investigations for the other end member KNbO 3 [21] of the solid solution sodium potassium niobate (Na,K)NbO 3 , we find that the improved GGA approximations of AM05 and PBEsol perform better for the structural, electronic, and optical properties compared to the conventional PBE approximation. e results will be beneficial for future theoretical works concerning strain influences from underlying substrates or calculations of the spontaneous polarisation.…”

Optical properties of titanium-doped lithium niobate from time-dependent density-functional theory

“…Our ground-state calculations are performed within densityfunctional theory as implemented in the QUANTUM ESPRESSO package [20]. For the exchange-correlation energy we use the PBEsol parametrization [21], a generalized gradient approximation that reliably reproduces the experimental lattice constants of solids, including LN and related materials [22][23][24]. The electron-ion interaction is modeled using optimized normconserving Vanderbilt pseudopotentials [25], with the 1s and 2s orbitals of lithium, the 2s and 2p orbitals of oxygen, the 4s, 4p, 4d, and 5s orbitals of niobium, and the 3s, 3p, 3d, and 4s orbitals of titanium treated as valence electrons.…”

Polaron optical absorption in congruent lithium niobate from time-dependent density-functional theory

“…The convergence threshold for the Hellmann-Feynman forces is set to 0.01 eV/Å. We use the modified Perdew-Burke-Ernzerhof parametrization for solids (PBEsol) [25] for the exchangecorrelation energy, a generalized gradient approximation that accurately reproduces the experimental lattice constants of solids, including LN and related materials [26][27][28].…”