Jahn—Teller deformations

SA quite different approach (Biirgi and Dunitz, 1983), using atomic vibration tensors to estimate torsional energy barriers (assumed sinusoidal) about single bonds, is described by Trueblood and Dunitz (1983) and has been applied to pseudo-Jahn-Teller deformations of octahedral complexes by Stebler and Biirgi (1987). 

The most interesting possibility indicated by this phase diagram is that the FO phase may be present as a metastable phase below Tjt = 26 K, instead of the HS phase above rJT. It suggests that physical properties of the metastable phase may change at rJT. Recently, a photo-induced metastable phase of an Fe(II) spin-crossover complex [Fe(2-pic)3]Cl2 EtOH (2-pic = 2-aminomethylpyridine) was reported [26-28]. Its properties are remarkably different from those of the metastable HS phase quenched by rapid cooling. Lowered symmetry accompanying the Jahn-Teller deformation has... 

One doesn t need a real degeneracy to benefit from this effect. Consider a nondegenerate two-level system, 84, with the two levels of different symmetry (here labeled A, B) in one geometry. If a vibration lowers the symmetry so that these two levels transform as the same irreducible representation, call it C, then they will interact, mix, and repel each other. For two electrons, the system will be stabilized. The technical name of this effect is a second order Jahn-Teller deformation.67... 

They indicate a static distortion with the symmetry of a tetragonal elongation. By raising the temperature the EPR signal becomes isotropic, which can be accounted for by the assumption of a dynamic Jahn-Teller deformation of the CUL5 octahedra. Obviously there is a rapid temperature-induced change of the preferred axis between the octahedral x-, y- and z-directions. This change has the symmetry of a pseudorota-... 

We will now consider the problem how to get information about the extent of the underlying Jahn-Teller deformation (pmm) Jahn-Teller effect is dynamic. Using ideas of Ehinitz and Orgel Ammeter et al. have proposed an interesting procedure to calculate approximate values for the radial distortion parameter Pmin ftoni the rms displacements in the case of dynamically distorted CuL octahedra l. They defme the Jahn-Teller contribution to the mean square amplitude U of the Cu—L stretching vibrations by [compare Eq. (5)] ... 

It should be noted fiiat die parameter A, responsible for the interaction of lanthanide ions via the phonon field, is always negative, and fire corresponding renormalization (eqs. 100 and 101) decreases the efficiency of the electronic subsystem interaction with the lattice. Correlations of the local (Jahn-Teller) deformations essentially affect the response of the lanthanide ions to an applied magnetic field only in the case of sufficiently strong interactions of those ions with macroscopic deformations. 

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