Calculation of strained BaTiO3 with different exchange correlation functionals examined with criterion by Ginzburg-Landau theory, uncovering expressions by crystallographic parameters

Examination of permittivity for depolarization field of ferroelectric by ab initio calculation, suggesting hidden mechanisms

Electrostatics of depolarization field Ed in relation to the polarization is studied. In particular, the value of permittivity for Ed (εd) in prototypical situations of ferroelectrics, including Mehta formula, is examined by ab initio calculations. By using spontaneous polarization PS corresponding to accurate experiment ones, we show εd = 1, which suggests that the results of εd ≫ 1 indicate hidden mechanisms; εd = 1 suggests that the effect of Ed is significant to induce intriguing important phenomena overlooked by εd ≫ 1. A bridge between εd = 1 and εd ≫ 1, i.e. the consistency of εd = 1 with conventional results is presented. The exact electrostatic equality of head-to-head-tail-to-tail domains to free-standing ferroelectrics is deduced. Hence, most stoichiometric clean freestanding monodomain ferroelectrics and head-to-head-tail-to-tail domains are shown unstable regardless of size, unless partially metallic. This verifies the previous results in a transparent manner. This conclusion is shown consistent with a recent hyperferroelectric LiBeSb and "freestanding" monolayer ferroelectrics, of which origin is suggested to be adsorbates. In addition, this restriction is suggested to break in externally strained ultrathin ferroelectrics. The macroscopic formulas of Ed are found valid down to a several unit-cells, when electronic and atomic-scale surface effects are unimportant and accurate PS is used.

Electron-electron correlations and structural, spectral and polarization properties of tetragonal BaTiO3

To analyze the role of electron-electron correlation effects in structural (local-geometry), spectral and polarization properties of tetragonal BaTiO3we apply DFT +Uapproach. We demonstrate that the system properties drastically change when the value of the local Coulomb repulsionUcrosses the critical valueUc≈ 7 eV. In particular, the correlation effects cause a change of the ratio of the in-plane and inter-plane Ti-O bond lengths, which results in a flip of the order of the Tid-bands and change of the polarizability of the system. Since the consensus value ofUin BaTiO3is unknown, we discuss how the obtained results may be revealed in experimental data, especially in the optical response and ultrafast charge dynamics, where effectiveUis dynamically tuned.