Jahn-Teller distortion, spin-orbit

It has been shown, however, that the orbital degeneracy of the 3Ti(F) state would be lifted by spin-orbit coupling 142). In this case there is no longer any need for Jahn-Teller distortion, and when the four ligands are identical the tetrahedral complexes may be quite regular. 

Liehr has shown (142) that the orbital degeneracy of the ground state in a tetrahedral nickel(II) complex may be lifted by spin-orbit coupling. This means that these complexes may not be liable to Jahn-Teller distortion as has been thought for some time. Such coupling would also have the effect of splitting all transitions into several components, the exact number... 

The final application of LFMM concerns a subtle yet unexpectedly significant Jahn-Teller effect. The classic examples of Jahn-Teller distortions are associated with d9and high-spin d4 systems with orbitally degenerate ground states of E symmetry... 

It is the purpose of this chapter to review the recent developments in this field with a particular emphasis on the interplay between orbital ordering of the Jahn-Teller-distorted C60ions and spin ordering leading to the ferromagnetic state. 

Transition metal ions most susceptible to large Jahn-Teller distortions in octahedral coordination in oxide structures are those with 3d4, 3d9 and low-spin 3(f configurations, in which one or three electrons occupy eg orbitals. Thus, the Cr2+ and Mn3+, Cu2+, and Ni3+ ions, respectively, are stabilized in distorted environments, with the result that compounds containing these cations are frequently distorted from type-structures. Conversely, these cations may be stabilized in distorted sites already existing in mineral structures. Examples include Cr2+ in olivine ( 8.6.4) and Mn3+ in epidote, andalusite and alkali amphiboles ( 4.4.2). These features are discussed further in chapter 6. 

The geometry of figure below arises from a Jahn-Teller distortion of the octahedron. The eg orbitals are split in energy and this permits stabilisation of a complex if these two orbitals are unequally occupied. Thus in d9 (Cu2+) two electrons occupy the dz2 and one the dx2, 2 Nearly all Cu2+ compounds show this type of distortion, as do many high-spin d4 ions such as Cr2+. 

Figure 2. (a) A-type antiferromagnetic spin order, (6) Jahn-Teller distortion pattern and (c) orbital ordering in LaMn03 (after Rao (2000)). 

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