Mutual Influence of Distortive, Magnetic, and Electric Dipole Orderings in Jahn—Teller Elastics

Abstract

One of the distinctive attributes of Jahn—Teller crystals is the fact that not only the interaction responsible for the ordering of local distortions and hence for the corresponding phase transition, but also interactions conducive to the ordering of magnetic moments, electric dipoles, or local distortions having a different symmetry can play a major role. The important mutual influence of these interactions is attributable to the presence of a group of lower energy levels (degenerate or nondegenerate), typical of Jahn—Teller ions, on whose basis the corresponding operators are defined. According to the way the interactions are related (magnitudes, commutative relations for the corresponding operators), phase transitions of one order or another or a phase with corresponding orderings of a different physical nature occurs in crystals. The present chapter deals with interactions between the orderings and their mutual influence. We note that this interaction is considered here for arbitrary relations between the interacting quantities. However, even when one of them prevails and a single phase transition occurs, interstitial correlations of some other nature have a strong effect on it. Another distinctive feature of Jahn—Teller crystals, namely the onset of various combinations of significant anomalies of various properties, is a consequence of the above interaction of distortive, electric dipole, and magnetic structures. In what follows we show that, in addition to the peculiar elastic anomalies considered in the preceding chapter, Jahn—Teller compounds are also characterized by very specific magnetic and dielectric properties. Combinations of the above properties depend on the particular type of structural ordering, whether of the ferro, antiferro, or ferri type. Various attendant physical situations are also discussed in this and subsequent chapters.