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In terms of the self-consistent phonon field method generalized with regard to the orientational degrees of freedom the dynamics of the formation of the mesomorphic state structure at the activation of the molecular crystal oscillations is studied. The reasons for the existence and for the stabi]ity of the partial]y ordered media with Uquid-erystalline and plastic packing of the molecules ate substantiated. The dependencies of the dynamical crystal characteristics versus temperature ate calcu]ated. From the analysis of the lihrationa] and translational anharmonicities evotutiou at the temperature aheration the energetic relations for the transition of the molecular crystal into the mesostate ate established. It is shown that the anharmonic processes bound-up with different degrees of the moleeules freedom gire ah essentially unequal contribution to the development of the dynamical instability of the systeta. The mutual correlation between the struetural instability type, the molecular crystalHne anisotropy coefficient and the molecular interaction anisotropy factor are investigated. The behaviour of the critical values as functions of the system anisotropy parameters is considered. Conc]usions ate drawn regarding the crystal lattiee disarrangement in the pretransitional regions of liquid-crystalline and plastie mesogencity. l. IntroductionFrom the microscopio point of view phase transitions in ordered structutes arise a~ a resu]t of the instability of the collective partic]es excitations [I--3]. The detai]ed microscopic picture of the phase transition may be obtained on the basis of the analysis of the systems dynamics on|y [4--6].The present paper is devoted to the deve]opment of the dynamical approach to the problem of the thermotropic mesomorphic state formation [7--11]. Due to the comp]exity of the molecular structure, packing, interaction and the steric effects, however, it is not possible to bui]d up the successive theory of the mo]ecu]ar systems dynamics. Therefore for the estab]ishment of the structura] instabi]ity mechanisms and for their detai]ed investigation the e]aboration of simple mode]s is of a great importance. According to [7,8] we sha]l consider a quasi-onedimensiona] molecular crysta] whose partir have trans]ationa] degrees of freedom a]ong some direction in the space and perform angular osci]]ations in the perpendicular planes. For c]earing up the ro]e ‰ the various types of the mo]ecu]es movement in the structura] insta]~i]ity ori 91 and for the determination of the critica] parameters of the molecular crysta] in the ]t/gh temperature ]imit it is necessary to undertake an ana]ysis of the Acta Physiaz Amdemiae Sr