Cooperative Jahn-Teller distortion

The crystalline anisotropy and magnetostriction are sensitive to the minority-spin electrons at Fe " ions, especially if this spin is localized and produces a cooperative Jahn-Teller distortion. The Fe ion contribution to these properties arises primarily from dipole-dipole Interactions. 

The high-temperature transport data has been well rationalized 0 on the basis of a diffusional model with AH = 0.16 eV for all x, which is an agreement with localized Fe " -ion configurations responsible for a cooperative Jahn-Teller distortion below Tn. The ratio (1 - c)/c obtained from Eq. (15) for the Seebeck coefficient, with a spin-degeneracy factor P = 2, gives the factor c(l — c) entering the conductivity expression... 

An effective Hamiltonian for a static cooperative Jahn-Teller effect acting in the space of intra-site active vibronic modes is derived on a microscopic basis, including the interaction with phonon and uniform strains. The developed approach allows for simple treatment of cooperative Jahn-Teller distortions and orbital ordering in crystals, especially with strong vibronic interaction on sites. It also allows to describe quantitatively the induced distortions of non-Jahn-Teller type. 

The Jahn-Teller effect is associated with electron-lattice interactions. In particular, when the electronic states of the nondistorted molecule are degenerate, the Jahn-Teller effect removes this degeneracy by distorting the molecule. Orbital ordering can also lead to a cooperative Jahn-Teller distortion and three-dimensional ferromagnetic ordering. 

In some cases, the above cubic to tetragonal transition is followed by other types, such as the Oh5— C 5—>C2fe3— >C2f 5 phase transitions of Cs2RbDyF6 [49, 54]. A first-order phase transition, involving a cooperative Jahn-Teller distortion has been reported for Cs2NaHoCl6 at low temperature, 150 mK [61]. 

The complex fluoride CrZrFe (Table 31) is an example of a class of compound A M Fs which undergoes phase transitions as the temperature changes. In the chromium(II) and copper(II) compounds additional modifications occur because of local and cooperative Jahn-Teller distortions. From the three spin-allowed d-d transitions for high-spin Cr in strongly distorted CrFe octahedra (Table 31) an octahedral A value of 7600 cm is calculated. Transitions between the monoclinic IT, pseudotetragonal I and cubic I phases have been... 

IV. Local and Cooperative Jahn-Teller Distortions in Nitrocomplexes AJM T (N02)6... 

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