Interaction of Jahn—Teller Centers

In Chapter 1 we have briefly considered the fundamentals of the Jahn—Teller effect and have discussed some of its manifestations for isolated Jahn—Teller centers. One of the main conclusions of Chapter 1 is that the Jahn—Teller effect does not induce static distortions. A distorted nuclear configuration does actually correspond to a certain electronic state of the degenerate term, but such distorted equilibrium configurations (potential energy minima) are not always unique, and dynamic averaging over them restores the initial high symmetry. On the other hand, it has been noted that even slight low-symmetry perturbations (external fields, random strains, etc.) in Jahn—Teller situations can tend to localize the system at one of the minima and bring about significant distortions. In this situation it would be interesting to know whether interaction between Jahn—Teller centers, as in the case, e.g., of a crystal with an ionic sublattice whose ground state is degenerate, can lead to localization and a net distortion. In such a multicenter Jahn—Teller situation it is also reasonable to expect phase transitions similar to those accompanying spin ordering, the only difference is that pseudospins specified in a basis of orbital states should be addressed in this case. The investigation of such phase transitions due to ordering of the orbital components of degenerate electronic states of lattice ions is the main purpose of this book. For phase transitions to occur, there must be interaction between electronic states of different sites. In this chapter we shall discuss the mechanisms of such interaction.