Ferroelectricity, vibronic theory

Our brief review of Bersuker s academic life won t be complete, without mention to his excellent reviews and books on the Jahn-Teller effect and vibronic interactions in molecules and solids, on vibronic theory of ferroelectricity, on electron properties of coordination compounds, and on the electron-conformational approach to the problem of identification of biologically active units in a large set of molecules. However, the most important creation of Isaac Bersuker is his school, the world-known group of scholars that hold his system of views and follow his high... 

Determination of the critical temperature after equation (1), T0 = En/Syl, is considered in the cooperative JT and PJT effects [2,3,8,9]. In particular, the origin of structural ferroelectric phase transitions as due to the PJT effect (the vibronic theory of ferroelectricity) was suggested first in the sixties [10] (see also Ref. [9]). JT structural phase transitions are reviewed in Ref. [8]. 

The well-known Landau-Ginsburg-Cochran-Anderson theory is a phenomenological approach. Of the first-principle theories, just one, the vibronic theory of ferroelectricity (e.g., see [1,51]), is consistently based upon electronic structure and chemical nature of the respective compounds. 

In the OOA, as it was discussed above, ligands are omitted. The vibrational motion of low-symmetry distortions is averaged out. Electron wave functions do not follow nuclear displacements. Therefore, in the OOA, the abovementioned crystal-lattice mechanism of spontaneous polarization is lost. The only possibility left in the OOA is polarization of electron wave functions. Without lattice distortion involved, this pure electron-shell mechanism was discussed in literature long ago, back about 45 years. Lacking supporting evidence from experimental data and electron-structure evaluation, it was rejected. On the contrary, the vibronic theory of ferroelectricity cumulated overwhelming experimental evidence. (For an updated review, see Sect. 8.3 in Bersuker s book [1].)... 

In the present review article 1985 s results obtained in applications of the concept of vibronic interactions to the investigation of electric properties of molecules (dipole and multipole moments and polarizabilities) are presented. Molecular aspects of these topics are almost untouched in the publications listed in the preceding paragraph. The idea of dipole instability was used first as a basis of the so-called vibronic theory of ferroelectricity (Bersuker, 1966 Bersuker and Vekhter, 1978). Meanwhile, the manifestation of the electronic or vibronic degeneracy in the electric responses of molecules, being no less essential than other vibronic effects, has some special features. 

B. Bersuker, Recent development of the vibronic theory of feroelectricity, Ferroelectrics 164, 75 (1995). 

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