Pseudo Jahn–Teller origin of ferroelectric instability in BaTiO3 type perovskites: The Green's function approach and beyond

Polinger, V. Z.; García‐Fernández, Pablo; Берсукер, И. Б.

doi:10.1016/j.physb.2014.09.048

Cited by 61 publications

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“…For better demonstrative purposes, we employ here the terminology of the simpler description in the cluster approximation considering the Ti centers in the octahedral environment of the six oxygens as the active units; [30][31][32] qualitatively similar results were obtained recently in a more sophisticated Green's functions approach 33 in which the Ti center interacts with the whole crystal via its electronic and vibrational band structure, and we use below these later results.…”

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confidence: 85%

“…To begin with, the criterion of PJTE polar instability (6) holds well for the Ti center in BaTiO 3 (Table I), 33 and it does not hold for the same ion in SrTiO 3 . The reason of this is straightforward: The size of the Sr 2þ ion that controls the unit cell dimensions is smaller, and hence, the interatomic Ti-O distance is smaller in the latter case, and this, in turn, leads to larger stiffness K 0 and smaller F value, both deteriorating the condition of instability (6).…”

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confidence: 99%

“…There may be many such excitations, so the problem is better formulated as the vibronic coupling of the six HOMOs t 1u and t 2u to the three LUMOs t 2g via the dipolar displacements Q x , Q y , and Q z . With the linear vibronic coupling included, the secular equation of the PJTE yields the following APES [30][31][32][33] (…”

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confidence: 99%

“…[30][31][32][33] For BaTiO 3 , the vibronic coupling parameters were estimated using the Green's function description 33 and two experimentally determined structural constants: the energy gap 2D ¼ 2.8 eV and the vibrational frequency at the bottom of the trigonal minimum hx ð ¼ 193 cm…”

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Pseudo Jahn–Teller effect in the origin of enhanced flexoelectricity

Берсукер

2015

Applied Physics Letters

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The controversy between the theory and experiment in explaining the origin of enhanced flexoelectricity is removed by taking into account the pseudo Jahn-Teller effect (PJTE) which, under certain conditions, creates local dipolar distortions of dynamic nature, resonating between two or more equivalent orientations. The latter become nonequivalent under a strain gradient thus producing enhanced flexoelectricity: it is much easier to orient ready-made dipoles than to polarize an ionic solid. For BaTiO3, the obtained earlier numerical data for the adiabatic potential energy surface in the space of dipolar displacements in the Ti centers were used to estimate the flexoelectric coefficient f in the paraelectric phase in a one-dimensional model with the strain gradient along the [111] direction: f = −0.43 × 10−6 Cm−1. This eliminates the huge contradiction between the experimental data of f ∼ μ Cm−1 for this case and the theoretical predictions (without the PJTE) of 3–4 orders-of-magnitude smaller values. Enhanced flexoelectricity is thus expected in solids with a sufficient density of centers that have PJTE induced dipolar instabilities. It explains also the origin of enhanced flexoelectricity observed in other solids, noticeable containing Nb perovskite centers which are known to have a PJTE instability, similar to that of Ti centers. The SrTiO3 crystal as a virtual ferroelectric in which the strain gradient eases the condition of PJTE polar instability is also discussed.

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confidence: 85%

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Pseudo Jahn–Teller effect in the origin of enhanced flexoelectricity

Берсукер

2015

Applied Physics Letters

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“…For BaTiO 3 in the paraelectric phase numeric estimates [5] show that the energy barrier between the minima of the APES is rather low, of the order of 200 cm -1 , but the vibrational frequency in the direction to the barrier is also low, so the zero-vibrations level is at about half the barrier height, at ~100 cm -1 . Hence we may expect the tunneling transitions between the minima at low temperatures being overwhelming with more intensive over-the-barrier transitions at higher temperatures.…”

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Pseudo Jahn-Teller effect in permittivity of ferroelectric perovskites

Polinger

Берсукер

2017

J. Phys.: Conf. Ser.

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Abstract. Microscopic origin of dielectric susceptibility and permittivity of cubic ferroelectric perovskites ABO 3 (BaTiO 3 , KNbO 3 , etc.) in their high-symmetry paraelectric phase is explored. Ferroelectricity in these crystals is due to the strong pseudo Jahn-Teller effect (PJTE) resulting in off-centre instability of the transition metal ion B. In addition to the noticed earlier big enhancement of the permittivity by the local PJTE, it is found that the latter induces also anisotropy, contrary to widespread expectation for isotropic polarization in cubic crystals. The temperature and field dependence of polarization, dielectric susceptibility, and permittivity, as well as their angular dependence, are revealed as resulting from the local PJTE instability by numerical diagonalization of the Hamiltonian including the vibronic coupling and temperatureaveraged interaction with the external electric field. IntroductionWe consider the dielectric susceptibility and permittivity of ferroelectric perovskites ABO 3 , like BaTiO 3 , KNbO 3 , etc., in which the polar instability of the elementary-cell octahedron [BO 6 ] is due to the pseudo Jah-Teller effect (PJTE) [1][2][3]. The adiabatic potential energy surface (APES) of this octahedron was shown to have eight equivalent wells situated along the four trigonal axes С 3 (figure 1), in which the atom B is off-center shifted making a local dipole moment, with potential barriers between the wells. At higher temperatures, in the paraelectric phase the local polar displacements in different centers have arbitrary (incoherent and dynamic) orientations that make the macroscopically averaged crystal seen as cubic. At a lower temperature T = T C, the ferroelectric phase transition results in orientation ordering of the local dipoles (partial or full) resulting in a polarized crystal, and the multi-valley APES explains the temperature-dependent formation of all the four ferroelectric phases [1].A detailed presentation of this vibronic coupling theory is given in a series of papers [1, 2] and books [3] including a recent more elaborate theoretical description based on the Green's functions approach [1]. Its main conclusions are confirmed in dozens of experimental observations forming a pattern (see one of them in [4] and many others cited in [1][2][3]). Recently the theory was expanded to also include the possibility of coexisting PJTE-induced ferroelectric and open-shell magnetic properties in perovskites ABO 3 , and a table was given listing the B atoms with electron configurations

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Spontaneous Symmetry Breaking in Matter Induced by Degeneracies and Pseudodegeneracies

Берсукер

2016

Advances in Chemical Physics

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Pseudo Jahn–Teller origin of ferroelectric instability in BaTiO3 type perovskites: The Green's function approach and beyond

Cited by 61 publications

References 86 publications

Pseudo Jahn–Teller effect in the origin of enhanced flexoelectricity

Pseudo Jahn–Teller effect in the origin of enhanced flexoelectricity

Pseudo Jahn-Teller effect in permittivity of ferroelectric perovskites

Spontaneous Symmetry Breaking in Matter Induced by Degeneracies and Pseudodegeneracies

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