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 ${\mathrm{CBr}}_{2}{\mathrm{Cl}}_{2}$ and ${\mathrm{CBrCl}}_{3}$ in their low-temperature monoclinic phases. The dielectric spectra display features which correspond to $\ensuremath{\alpha}$- and $\ensuremath{\beta}$-relaxation processes, commonly observed in canonical glass formers. NQR experiments, also performed in the ergodic monoclinic phase of ${\mathrm{CCl}}_{4}$, enable the determination of the microscopic mechanism underlying the $\ensuremath{\beta}$ 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 $\ensuremath{\beta}$-relaxation behavior, raising some doubt about the existence of a universal explanation of this phenomenon.