The Cooperative Jahn-Teller Effect

This general feature of both class I and class II behaviour of the copper(II) ion, i.e. the ability to exist in a high symmetry environment against the prediction of both the first- and second-order Jahn-Teller effects, is the best single piece of evidence for the cooperative Jahn-Teller effect and has recently been reviewed.432 It is generally responsible for the whole range of fluxional copper(II) stereochemistries and of the temperature variable ESR spectra of... 

Orbital Ordering and the Cooperative Jahn-Teller Effect in Single Crystals of the Magnetic Perovskite La7/8Sr1/8Mn03... 

It is generally accepted in the theory of the cooperative Jahn-Teller effect to include the interaction with uniform strains in the way proposed by Kanamori [14], i.e., as additional terms of vibronic interaction at each Jahn-Teller ion. On the other hand, within one-centre-coordinate approach used here the vibronic interaction is fully described by means of one-centre active nuclear displacements qn. Therefore the interaction with uniform strains can be included implicitfy as additional terms in the Van Vleck expansion (3). Since phonons and uniform strains are independent degrees of freedom this new expansion is written as follows ... 

Lattice vibrations tend to destroy the correlation among Jahn-Teller centers. Thus, with increasing temperature, these centers may become independent of each other at a certain point, and their static Jahn-Teller effects convert to dynamic ones. At this point the crystal as a whole becomes more symmetric. This temperature-dependent static -O- dynamic transition is called a Jahn-Teller phase transition. Below the temperature of the phase transition, the cooperative Jahn-Teller effect governs the situation providing static distortion the overall structure of the crystal is of a lower symmetry. Above this temperature, the cooperation breaks down, the Jahn-Teller distortion becomes dynamic and the crystal itself becomes more symmetric. 

Thus in fulleride solids, depending on the interplay of several parameters, multiple phases are possible and phase transitions can occur when varying the cation size or the temperature or pressure. Dunn [11] has investigated these effects in detail for the cooperative Jahn-Teller effect in solids and gave a general description for icosahedral systems. 

The exact temperature of the phase transition in the experiments above is subject to some uncertainty, which we attribute to the stoichiometric variations in both the central atom of the counterion (P,As) and the type of the halide. The picture that emerges, however, is compatible with the full scope of experimental results a dynamic Jahn-Teller effect at high temperature, a structural phase transition in the 120-150 K range, and a distortion of the molecular ion in the low-temperature phase, arising from the positive synergy of the cooperative Jahn-Teller effect and the low symmetry of the environment. The static-to-dynamic transition does not coincide with the structural transition and its temperature depends on the detection method whereas ESR spectra are isotropic at room temperature, infrared lines are still split, indicating a distorted state on the time scale of the measurement. This... 

Quasidynamical Model for the Cooperative Jahn-Teller Effect. Vibronic Intervalence Optical Bands... 

Abstract Keeping in mind the pedagogical goal of the presentation the first third of the review is devoted to the basic definitions and to the description of the cooperative Jahn-Teller effect. Among different approaches to the intersite electron correlation in crystals the preference is with the most fundamental and systematic Hamiltonian shift transformation method. Order parameter equations and their connection to the crystal elastic properties and to the orbital ordering are considered. An especial attention is paid to the dynamics of Jahn-Teller crystals based on the coupled electronic, vibrational, and magnetic excitations which are of big interest nowadays in orbital physics. 

The cooperative Jahn-Teller effect (CJTE) as a viable mechanism of structural transformations in solids was mentioned for the first time in 1957 by Dunitz and Orgel [ 1 ] and McClure [2]. Relatively soon after that Kanamori [3] published a fundamental paper defining the basic approach to the problem. In 1966 Bersuker [4] suggested a Jahn-Teller model for formation an electric dipole moment of a molecule... 

Orbital Ordering Versus the Traditional Approach in the Cooperative Jahn-Teller Effect A Comparative Study... 

The Cooperative Jahn-Teller Effect and Orbital Ordering... 

The local Jahn-Teller effect in isolated octahedra is the subject of the next chapter. Experimental examples of Cu doped into compounds of perovskite and elpasolite structure wiU be given. In Chap. Ill the cooperative Jahn-Teller effect as the consequence of high concentrations of Jahn-TeUer centers in a solid matrix will be considered. In Chap. IV we discuss as illustrative model examples the highly interest-... 

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