Jahn-Teller effect

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Jahn-Teller effect

A distortion of a highly symmetrical molecule, which reduces its symmetry and lowers its energy. The effect occurs for all nonlinear molecules in degenerate electronic states, the degeneracy of the state being removed by the effect. It was first predicted in 1937 by H. A. Jahn and E. Teller. In early experimental work, the effect often “disappeared” or was masked by other molecular interactions. This has surrounded the Jahn-Teller effect with a certain mystery and allure, rarely found in science today. However, there are now a number of clear-cut experimental examples which correlate well with theoretical predictions. These examples range from the excited states of the most simple polyatomic molecule, H3, through moderate-sized organic molecules, like the ions of substituted benzene, to complex solid state phenomena involving crystals or localized impurity centers. See Crystal defects, Degeneracy (quantum mechanics), Molecular structure and spectra, Quantum mechanics

With the exception of linear molecules which suffer Renner-Teller effects, all polyatomic molecules of sufficiently high symmetry to possess orbitally degenerate electronic states will be subject to the Jahn-Teller instability. However, in cases other than molecules with fourfold symmetry, the proof is somewhat involved and requires the use of the principles of group theory. See Renner-Teller effect

Jahn-Teller effect

[′yän ′tel·ər i‚fekt]
(physical chemistry)
The effect whereby, except for linear molecules, degenerate orbital states in molecules are unstable.
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It is widely recognized that one of the reasons for the lack of understanding of these systems is that several degrees of freedom are intimately interconnected and that the identification of the leading interaction is a challenging task, a typical example being the interplay between orbital ordering (OO) and cooperative Jahn-Teller distortion (cJTd).
The insertion of Mg into the crystal lattice and the relaxation of the position of fluorine atoms have no relevant effects onto the population of the d orbitals of copper, suggesting that (i) the perturbation due to Mg doping is extremely localized, (ii) slight modifications of the Jahn-Teller distortion around Cu have little effect on the orbital ordering, and therefore (iii) the Jahn-Teller distortion and the orbital ordering have different energy scales.
EPR and XAFS show Jahn-Teller distortion around copper in the complex.
The copper complex also shows a Jahn-Teller distortion. XAFS analysis was done on the copper complex and XAFS confirmed the above stated results obtained by other techniques.
The maps clearly showed how the presence of oxygen atoms forced the atoms into a checkerboard pattern known as a Jahn-Teller distortion. Gai says the team s study is the first time the phenomenon has been observed on a material s surface.
Therefore Cu and Zn are mutually replaceable and are surrounded by oxygen molecules in the oxide form, since [Cu.sup.2+], being a [d.sup.9] system, is prone to Jahn-Teller distortion. Hence a EPR study of these ions will provide information about the type of distortion and nature of bonding between metal and ligand.
(3) EPR results suggest that the nano CuZn[O.sub.2] compound Cu(II) ion has undergone Jahn-Teller distortion and is in the tetragonal elongated symmetrical environment which is further supporting the XRD results.
This severe capacity fading is mainly due to the Jahn-Teller distortion on the surface of spinel Li[Mn.sub.2][O.sub.4] [4], the dissolution of manganese in the electrolyte solution [5, 6], the spinel Li[Mn.sub.2][O.sub.4] with oxygen deficiency [7], and the decomposition of electrolyte solution on the electrode [8, 9].
Moreover, spinel [Li.sub.1.02][Mn.sub.1.92][Al.sub.0.02][Fe.sub.0.02][Cr.sub.0.02][O.sub.3.92][F.sub.0.08] shows better cycle performance and a slightly higher capacity than the spinel [Li.sub.1.02][Mn.sub.1.92][Al.sub.0.02][Fe.sub.0.02][Cr.sub.0.02][O.sup.4] because the substitution of Mn by Cr, Fe, Al, and Li decreases the unit cell volume and the decrease of [Mn.sup.3+] concentration reduces the Jahn-Teller distortion and also stabilizes the structure integrity of the active.
When these are doped with divalent ion, their resistivity decreases with formation of [Mn.sub.+4], which decreases the Jahn-Teller distortion, creates double exchange interactions, and hence plays a crucial role in the electrical transport and magnetic properties of these oxides [1].
The differences in the Ni-N and Cu-N bond lengths could be attributed to Jahn-Teller distortions for the [(e).sup.4][([t.sub.2]).sup.4] and [(e).sup.4][([t.sub.2]).sup.5] electronic configurations in the respective Ni-1MeIm and Cu-1MeIm complexes [27].
These differences in the energy support the Jahn-Teller distortions observed in Table 1 as a result of unequal occupancy of the [t.sub.2] orbitals in a tetrahedral field.