Jahn-Teller phase transition

Lattice vibrations tend to destroy the correlation among Jahn-Teller centers. Thus, with increasing temperature, these centers may become independent of each other at a certain point, and their static Jahn-Teller effects convert to dynamic ones. At this point the crystal as a whole becomes more symmetric. This temperature-dependent static -O- dynamic transition is called a Jahn-Teller phase transition. Below the temperature of the phase transition, the cooperative Jahn-Teller effect governs the situation providing static distortion the overall structure of the crystal is of a lower symmetry. Above this temperature, the cooperation breaks down, the Jahn-Teller distortion becomes dynamic and the crystal itself becomes more symmetric. 

The temp>erature of the Jahn-Teller phase transition is very high for CuFj, CuClj, and KjCuF among the examples mentioned above [6-31]. Therefore, at room temperature their crystal structures display distortions. Other compounds have symmetric crystal structures at room temperatures as their Jahn-Teller phase transition occurs at lower temperatures. Cupric chelate compounds and [CuCNOj), ] compounds, such as KjPbCuCNOj) and TljPbCuCNO ), can be mentioned as examples [6-33]. Further cooling, however, may make even these structures distorted [6-33]. 

The presence of conduction electrons can lead to an additional quadrupolar coupling of the RE-ion. In addition they will lead to damping of the collective excitations due to possible electron-hole excitations. It will be demonstrated that the damping of sound waves due to that mechanism is particularly large in the vicinity of a Jahn-Teller phase transition. 

In the vicinity of a Jahn-Teller phase transition one finds... 

The excitation spectrum is determined from the poles of D (q, energy states excitations). The latter enter via the frequency dependent susceptibility q V, interaction between both types of modes leads to a hybridization (Elliott et al., 1972). Due to the elastic scattering processes within a CEF-energy level there are 5 =o-contributions to the quadrupole susceptibility (see eq. 17.108). It was shown that these processes have to be included in the expressions for the elastic constants and therefore contribute to a Jahn-Teller phase transition. However they do not show up in the acoustic phonon dispersion which one would measure... 

Figure 31 shows the proposed Longuet-Higgins loop for the cyclopentadienyl cation. It uses the type-VI Ai anchors, with the type-VII B structures as transition states between them. This situation is completely analogous to that of the radical (Fig. 23). Since the loop is phase inverting, a conical intersection should be located at its center—as required by the Jahn-Teller theorem.

A hysteresis cycle in the molar susceptibility measurements has been observed for [Ni2(Medpt)2(N3)2(/r-N3)2] (883). This has been ascribed to a phase transition caused by an asymmetrization process of the rhombus-like centrosymmetric [Ni-(N3)2-Ni] core that occurs with falling temperature. The asymmetrization transition can be explained in terms of a second-order Jahn-Teller distortion, taking into account the local symmetry of the dinuclear [Ni-(N3)2-Ni] entity (D2h, rhombic symmetry) before the arrangement.2128... 

An interesting aspect of many structural phase transitions is the coupling of the primary order parameter to a secondary order parameter. In transitions of molecular crystals, the order parameter is coupled with reorientational or libration modes. In Jahn-Teller as well as ferroelastic transitions, an optical phonon or an electronic excitation is coupled with strain (acoustic phonon). In antiferrodistortive transitions, a zone-boundary phonon (primary order parameter) can induce spontaneous polarization (secondary order parameter). Magnetic resonance and vibrational spectroscopic methods provide valuable information on static as well as dynamic processes occurring during a transition (Owens et ai, 1979 Iqbal Owens, 1984 Rao, 1993). Complementary information is provided by diffraction methods. 

There is agreement over the gross structural features of these linear chain chromium(II) complexes (and the isomorphous copper(II) analogues), but different models have been used for structural refinement. Phase transitions associated with the Jahn-Teller distortion (or to fi to y as the temperature is lowered) complicate the structural studies. 

Jahn-Teller induced symmetry breakings in structural phase transitions,... 

JAHN-TELLER INDUCED SYMMETRY BREAKINGS IN STRUCTURAL PHASE TRANSITIONS, MELTING, VAPORIZATION, AND ENANTIOMER FORMATION... 

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