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

Abstract

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.

DFT + U accurate for strain effect and overall properties of perovskite oxide ferroelectrics and polaron

“…Furthermore, the effects of temperature and quantum fluctuation are trivial compared with those of mechanical strain [19][20][21][22][23][24]40,41 and an unscreened depolarization field. 28,74 For example, epitaxial strain retains a BTO RT phase down to 4 K in experiments. 22 Hence, in many cases, including the calculation of RT properties, it is logically and practically preferable for the criterion of accuracy for DFT methods to be in agreement with experiments at 0 K. Because these experiments on FEs are rare, we proposed a process for deducing 0 K properties from experiments above 0 K 42 and used these 0 K properties as the accuracy criterion (Fig.…”