Symmetry, instability, stereochemistry and phase transitions in coordination compounds
A general model is worked out aomprising direct correlations between symmetry, vibronic instability,ster eo- chemistry and structural phase transformations in condensed media. The different phases of the latter are shown to oc- cur as a result of consequent sudden breaks of symmetry with transitions from the higher symmetry to the lower one by de- crease of temperature, the micromechanism triggering all these transformations being the same: the Jahn-Teller (pse- udo Jahn-Teller) effect. In addition to the previous results, it is shown rigorously that the Jahn-Teller effects a re the only possible sources of dynamic instability of high symme- try polyatomic systems with respect to lower symmetry dis- tortions, and thus these effects control all the stereoche- mica1 and structural transformations of matter. The visual reason of instability is due to formation of new covalent bonda (strong effect) and polarization of surrounding atoms (small effect) by distortion. Besides the vibronic origin of structural phase transitions in crystals and the transition liquid-crystal (melting) considered aarlier, the electroni- cally controled vibronic nature of the transition gas - liquid is discussed.