Özet:

For the first time the negative values of the Poisson ratios  have been calculated and the auxetic directions of fullerite  have been determined. The values of the Poisson ratios    are directed along the crystallographic directions of <110> type, which is indicative of the non-axial auxeticity. It has been established that with a rise in temperature from 100 K to 170 K, the degree of auxeticity  is reduced in proportion to reduction of elastic anisotropy factor . The auxeticity of fullerite  disappears completely in the temperature range of , the anisotropy remaining practically constant (А = 2.2). At  the degree of auxeticity again quickly grows in proportion to a rapid growth of anisotropy А. The revealed regularity is typical for crystals of all types of cubic crystal system: increase in the degree of auxeticity close to phase transition points. Such abnormal behaviour of  and  has been explained on the basis of analysis of thermodynamic properties of fullerite . In the high-temperature region , due to thermal excitation of the rotational degrees of freedom, the chaotic rotation of molecules  is increased, providing for the energy efficiency and stability of a closely packed face-centered cubic structure. It has been established that orientation glass-ordered phase transition is caused by a jump-like change in crystal lattice period  of fullerite , leading to increase in the degree of auxeticity close to . The mechanisms and regularities of auxeticity formation at points of phase transitions  and  have been revealed. It has been established that the Gruneisen parameter which is a degree of shift of crystal vibration spectrum frequencies    due to a change in the volume is more responsive to crystal lattice deformations. The method proposed here for determination of abnormal deformations with a change in rotational motion of molecules makes it possible not only to determine the appearance of auxetic properties, but also to get their quantitative value – the Poisson ratio    in certain crystallographic directions (   at ). Author Biographies Микола Дмитрович Раранський, Yurіy Fedkovych Chernіvtsi natіonal unіversity 2 Kotsyubynskogo str., Chernivtsi, Ukraine, 58012 Professor, Doctor of physical and mathematical sciences. Department of Solid State Physics

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