JAHN-TELLER EFFECT IN SOLID-STATE CHEMISTRY OF TRANSITION-METAL COMPOUNDS

The general features of the local and cooperative Jahn-Teller effect of dn cations with Eg ground states in octahedral coordination and simple model structures are discussed. Examples of different cooperative Jahn-Teller ordering patterns and of phase transitions from static to partially dynamic and finally fully dynamic Jahn-Teller determined structures are given. While in general a tetragonally elongated coordination of the Jahn-Teller ions is favored, the compressed configuration may be stabilized under certain conditions also. It is demonstrated by some examples that the extent of the Jahn-Teller distortion depends on the symmetry and connection pattern of the polyhedra in the host lattice structure. Finally it is shown that the crossover between high- and low-spin configurations of d7 cations is strongly influenced by the additional Jahn-Teller stabilization of the low-spin 2Eg state. © 1979.