Pseudo-Jahn–Teller Effect—A Two-State Paradigm in Formation, Deformation, and Transformation of Molecular Systems and Solids

Берсукер, И. Б.

doi:10.1021/cr300279n

Cited by 476 publications

(616 citation statements)

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“…Together with the well-known JTE as the source of instability of systems in degenerate states, this leads to the two-state paradigm: at least, two electronic states must be involved in SSB in polyatomic systems. 27 To demonstrate how the PJTE affects the flexoelectricity, we note first that-similar to the simplest case of two equivalent minima noted above-in any SSB, there are two or more equivalent directions of distortions with possible dynamics of the system resonating between them. In crystals, if the PJTE-produced broken-symmetry configurations at its centers are dipolar, their sufficiently strong interaction may result in ferroelectric phases at lower temperatures, or remain weakly-interacting centers with resonating, dynamically disordered dipoles.…”

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confidence: 95%

“…In treating the lattice-strain interaction, the papers above do not take into account the details of the local electronic structure and vibronic coupling that under certain conditions produce a local dipolar instability and dipolar distortions, which in the free system are of dynamic nature, but become oriented by the gradient of strain. The specific vibronic coupling effect that leads to dipolar instability in the dielectric systems under consideration is the PJTE 26,27 (regrettably, so far, this effect remains unknown to many solid state researchers).…”

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confidence: 99%

“…The latter is the driving force in the spontaneous symmetry breaking via the PJTE, triggering the local dipolar instability that produces the ferroelectric polarization of the crystal. 27 With the fully occupied HOMO, the ground state is A 1g . Since the three polar normal coordinates of the Ti center, Q x , Q x , and Q x , transform as t 1u , the general PJTE vibronic coupling problem is (A 1g þ T 1u ) t 1u , where T 1u is the excited electronic state formed by the one-electron excitation HOMO !…”

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confidence: 99%

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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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Pseudo Jahn–Teller effect in the origin of enhanced flexoelectricity

Берсукер

2015

Applied Physics Letters

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“…Successive snapshots of the carbon nanotube optimization process are shown in Figure 2, depicting the transition from the tubular structure, which has a partial sp 3 character, to the planar and purely sp 2 nanoribbon. To understand why these silicon and germanium structures are stable while the carbon one is not, we must consider the pseudo Jahn-Teller Effect (PJTE) [26][27][28] . Since silicon and germanium p orbitals are much closer in energy than the corresponding carbon ones 27 , the PJTE is much stronger in silicon and germanium structures than on their carbon counterparts.…”

Section: Resultsmentioning

confidence: 99%

One-dimensional silicon and germanium nanostructures with no carbon analogues

Perim

Paupitz

Botari

et al. 2014

Phys. Chem. Chem. Phys.

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In this work we report new silicon and germanium tubular nanostructures with no corresponding stable carbon analogues. The electronic and mechanical properties of these new tubes were investigated through ab initio methods. Our results show that the structures have lower energy than their corresponding nanoribbon structures and are stable up to high temperatures (500 and 1000 K, for silicon and germanium tubes, respectively). Both tubes are semiconducting with small indirect band gaps, which can be significantly altered by both compressive and tensile strains. Large bandgap variations of almost 50% were observed for strain rates as small as 3%, suggesting possible applications in sensor devices. They also present high Young's modulus values (0.25 and 0.15 TPa, respectively). TEM images were simulated to help the identification of these new structures.

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“…5 For traditional proper FEs, such as BaTiO 3 , the transition metal ion Ti 4+ with d 0 configuration can hybridize with the oxygen 2p states leading to the FE phase transition due to the pseudo Jahn-Teller effect. [6][7][8][9][10] However, FEs are very few in nature. 11 To find more high-performance FEs, improper (including hybrid improper) FEs become an intense research field.…”

Section: Introductionmentioning

confidence: 99%

Designing new ferroelectrics with a general strategy

Xiang

2017

npj Quant Mater

107

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The presence of a switchable spontaneous electric polarization makes ferroelectrics ideal candidates for the use in many applications such as memory and sensors devices. Since known ferroelectrics are rather limited, finding new ferroelectric materials has become a flourishing field. One promising route is to design the improper ferroelectrics. However, previous approach based on the Landau theory is not easily adopted for systems that are unrelated to the Pbnm perovskite structure. To this end, we develop a general design rule that is applicable to any system. By combining this rule with the density functional theory calculations, we identify previously unrecognized classes of ferroelectric materials. It is shown that the R3c perovskite structure can become ferroelectric by substituting half of the B-site cations. Compound ZnSrO 2 with a non-perovskite layered structure can also be ferroelectric through the anion substitution. Moreover, our approach can be used to design new multiferroics as illustrated in the case of fluorine substituted LaMnO 3 . 13 where the FE buckling (P mode) of the Y-O planes is induced by the non-polar MnO 5 polyhedra tilt (Q mode). 13,14 The free energy expansion in this system contains the coupling term (PQ 3 ) between the Q and P, indicating that the non-polar distortion Q must be reversed. 15 The hybrid improper ferroelectricity (HIF) was recently discovered in the artificial superlattice PbTiO 3 /SrTiO 3 , 16 where the ferroelectricity is induced by a trilinear coupling (PQ 1 Q 2 ) between the FE mode (P) and two oxygen octahedral rotational modes (Q 1 and Q 2 , respectively). The HIF was also found in the double-layered Ruddlesden-Popper (RP) perovskite A 3 B 2 O 7 (A=Ca, Sr; B=Mn,Ti), 17-19 the 1:1 A-cation ordering perovskite-type superlattice, 20-23 A-cation ordering RP NaRTiO 4 (R = Y, La, Nd, Sm-Ho), 24 the 2:2 B-cation ordered superlattice, 25 and metal-organic perovskite material. 26 In the above-mentioned theoretically designed FEs, one usually starts from a high-symmetry structure (e.g., cubic perovskite structure), then the effect of atomic substitution and soft phonon modulations are examined to see whether the ferroelectricity can be induced or not. Finally, the trilinear coupling mechanism is discussed to understand the origin of improper ferroelectricity. This procedure is indeed informative. However, it is tedious and its applicability to other type of compounds is limited since even the

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Pseudo-Jahn–Teller Effect—A Two-State Paradigm in Formation, Deformation, and Transformation of Molecular Systems and Solids

Cited by 476 publications

References 234 publications

Pseudo Jahn–Teller effect in the origin of enhanced flexoelectricity

Pseudo Jahn–Teller effect in the origin of enhanced flexoelectricity

One-dimensional silicon and germanium nanostructures with no carbon analogues

Designing new ferroelectrics with a general strategy

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