Electron-lattice interplay in 
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 from canonical Jahn-Teller distortion notations

Schmitt, Michaël; Zhang, Yajun; Mercy, Alain; Ghosez, Philippe

doi:10.1103/physrevb.101.214304

Cited by 16 publications

(11 citation statements)

References 92 publications

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“…129 The physical effect that will stabilize an insulating state is Q + 2 distortions: The deviation from 1 of the Goldschmidt tolerance factor can lead to different energy lowering reconstruction resulting in a change of electronic properties. For the case of LaMnO3, the compound exhibits spontaneous Q + 2 distortions 127,130,131 resulting in band gap opening (see Fig. 10c,d).…”

Section: Local Structural Motifs: Enabling Energy Lowering Pseudo-jah...mentioning

confidence: 99%

False metals, real insulators, and degenerate gapped metals

Malyi

Zunger

2020

Applied Physics Reviews

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As science and technology of conductors have transitioned from studying simple elemental metals such as Al or Cu to compound conductors such as binary or ternary oxides and pnictides, a special class of degenerate but gapped metals has been noticed. Their presumed electronic configurations show the Fermi level inside the conduction band or valence band, yet there is an "internal band gap" between the principal band edges. The significance of this electronic configuration is that it might be unstable towards the formation of states inside the internal band gap when the formation of such states costs less energy than the energy gained by transferring carriers from the conduction band to these lower energy acceptor states, changing the original (false) metal to an insulator. The analogous process also exists for degenerate but gapped metals with Fermi energy inside the valence band, where the energy gain is defined by transfer of electrons from the donor level to the unoccupied part of the valence band. We focus here on the fact that numerous electronic structure methodologies have overlooked some physical factors that could stabilize the insulating alternative, predicting instead false metals that do not really exist (note, this is in general not a physical phase transition but a correction of a previous error in theory that led to a false prediction of a metal). Such errors include: (i) ignoring spin symmetry breaking, such as disallowing magnetic spin ordering in CuBi2O4, or disallowing the formation of polymorphous spin networks in paramagnetic LaTiO3 and YTiO3; (ii) ignoring structural symmetry breaking, e.g., not enabling energy-lowering bond disproportionation (Li-doped TiO2, SrBiO3, or rare-earth nickelates), or not exploring pseudo-Jahn-Teller-like distortions in LaMnO3 or disallowing spontaneous formation of ordered vacancy compounds in Ba4As3; (iii) ignoring spin-orbit coupling (SOC) forcing false metallic states in CaIrO3 and Sr2IrO4. Some of these modalities lead to intermediate bands inside the gap that might trap carriers. The distinction between false metals vs real insulators is important because, (a) predicting theoretically that a given compound is metal even though it is found to be an insulator often creates the temptation to invoke a high order novel physical effects (such as correlation in d electron Mott insulators) to explain what was in effect caused by a more mundane artifact in a lower-level mean-field band theory, (b) recent prediction of exotic physical effects such as topological semimetals were unfortunately based on the above compounds that were misconstrued by theory to be metal, but are now recognized to be stable insulators not hosting exotic effects, and (c) practical technological application based on stable degenerate but gapped metals such as transparent conductors or electrides for catalysis must rely on the systematically correct and reliable theoretical classification of metals vs insulators.

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Section: Local Structural Motifs: Enabling Energy Lowering Pseudo-jah...mentioning

confidence: 99%

False metals, real insulators, and degenerate gapped metals

Malyi

Zunger

2020

Applied Physics Reviews

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“…Typically, any d-orbital JT distortion will have some planar rhombic (Q 2 ) or tetragonal (Q 3 ) character. However, sometimes when the degeneracy occurs in the t 2g orbital, there may instead be a trigonal component to the symmetry of the distortion, which manifests as an angular distortion instead (Child & Roach, 1965;Bacci et al, 1975;Holland et al, 2002;Halcrow, 2009;Teyssier et al, 2016;Schmitt et al, 2020;Streltsov et al, 2022). For the more commonly studied case of a degeneracy in the e g orbitals, the effect of a rotation of similarly changes the symmetry of the d orbitals.…”

Section: Figurementioning

confidence: 99%

“…This is important because JT distortions typically follow a particular symmetry. When the distortion is due to degeneracy in the e g orbitals it will be of E g symmetry; when it is due to degeneracy in the t 2g -degenerate orbitals it may be either E g or T 2g symmetry (Child & Roach, 1965;Bacci et al, 1975;Holland et al, 2002;Halcrow, 2009;Teyssier et al, 2016;Schmitt et al, 2020;Streltsov et al, 2022), although there is relatively little unambiguous experimental evidence for a JT-induced shear compared with more typical E g distortion.…”

Section: Introductionmentioning

confidence: 99%

Van Vleck analysis of angularly distorted octahedra using VanVleckCalculator

Nagle-Cocco,

Dutton

2024

J Appl Crystallogr

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Van Vleck modes describe all possible displacements of octahedrally coordinated ligands about a core atom. They are a useful analytical tool for analysing the distortion of octahedra, particularly for first-order Jahn–Teller distortions, but determination of the Van Vleck modes of an octahedron is complicated by the presence of angular distortion of the octahedron. This problem is most commonly resolved by calculating the bond distortion modes (Q 2, Q 3) along the bond axes of the octahedron, disregarding the angular distortion and losing information on the octahedral shear modes (Q 4, Q 5 and Q 6) in the process. In this paper, the validity of assuming bond lengths to be orthogonal in order to calculate the Van Vleck modes is discussed, and a method is described for calculating Van Vleck modes without disregarding the angular distortion. A Python package for doing this, VanVleckCalculator, is introduced and some examples of its use are given. Finally, it is shown that octahedral shear and angular distortion are often, but not always, correlated, and a parameter η is proposed as the shear fraction. It is demonstrated that η can be used to predict whether the values will be correlated when varying a tuning parameter such as temperature or pressure.

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“…Although we address the dynamics of electron transfer from isolated Jahn-Teller ions, we incorporate cooperative effects, known to be relevant in solids [39,44,[56][57][58][59][60][61]. The reason is that the dynamics of ions is much slower than the electronic transfer rates, so that we assume that cooperative effects restrict the possible Jahn-Teller deformations of the neighboring sites where the transferred electron can jump into (see Fig.…”

Section: B Cooperative Jahn-teller Effectsmentioning

confidence: 99%