New Microscopic Mechanism for Secondary Relaxation in Glasses

Abstract

The dynamics of simple molecular systems showing glassy properties has been explored by dielectric spectroscopy and nuclear quadrupole resonance (NQR) on the halogenomethanes CBr2Cl2 and CBrCl3 in their low-temperature monoclinic phases. The dielectric spectra display features which correspond to alpha- and beta-relaxation processes, commonly observed in canonical glass formers. NQR experiments, also performed in the ergodic monoclinic phase of CCl4, enable the determination of the microscopic mechanism underlying the beta dynamics in these simple model glasses: Molecules that are nonequivalent with respect to their molecular environment perform reorientational jumps at different time scales. Thus our findings reveal another mechanism that can give rise to typical beta-relaxation behavior, raising some doubt about the existence of a universal explanation of this phenomenon.

The plastic and liquid phases of CCl3Br studied by molecular dynamics simulations

“…Compounds with lower symmetry (C 3v or C 2v ) have attracted so far some attention from the experimental groups 12,[18][19][20] , but very little work has been done from a computational point of view. 21,22 The polymorphism of CCl 3 Br (C 3v molecular symmetry) has been studied by several methods such as calorimetry 18 and neutron scattering, 19 X-ray diffraction and densitometry 20 .…”

“…Tetrahalomethanes with T d molecular symmetry have been the subject of many experimental and computational studies [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] . Compounds with lower symmetry (C 3v or C 2v ) have attracted so far some attention from the experimental groups 12,[18][19][20] , but very little work has been done from a computational point of view.…”

Emergence of glassy-like dynamics in an orientationally ordered phase

“…27,55 In the present system the statistical disorder due to the fractional occupancy of the oxygen atom of the only molecule in the asymmetric unit of phase II gives rise to a clear primary relaxation with a considerable strength (see Fig. 3).…”

“…It is thus evident that different scenarios giving rise to the emergence of secondary relaxations are possible so then "there seems to be no universal microscopic mechanism describing them." 27 The present work discusses the dynamics of an orientationally ordered (OO) low-temperature phase displaying statistical disorder, so yet another case for which the secondary relaxation could be ascribed to a different physical origin. The material studied is 2-adamantanone (C 10 H 14 O, hereinafter called 2A=O).…”

“…25,26 By decreasing still more the complexity of the system, the emergence of the α-and β-relaxation scenario was found even in systems with ordered phase and ascribed to a new microscopic mechanism. 27 It concerns tetrahedral molecules (CBr n Cl 4−n , n = 0, 1, 2) forming a low-symmetry lattice that are nonequivalent with respect to their molecular environment. More specifically, the monoclinic low-temperature ordered phases of such materials contain Z = 32 molecules with an asymmetric unit formed by Z = 4 molecules and an intrinsic disorder with respect to the occupancy of the halogen sites for molecules with n = 1 and n = 2.…”