Dynamics of an ergodic to non-ergodic phase transition within a long-range ordered phase

J. LL. TAMARIT; L.C.PARDO; M. ZURIAGA; P. LUNKENHEIMER; N. VEGLIO; F.J. BERMEJO; M. BARRIO; A. LOIDL

Università di Roma Rome, Italy

Conferencia; 6th International Discussion Meeting on Relaxations in Complex Systems; 2009

According to the original work of Johari and Goldstein [1], secondary relaxation processes not coming from intramolecular relaxations are know as a universal feature of disordered phases. In general, such β-processes are assigned to regions with lower density where part of the molecules are located [2] or to small-angle reorientations performed by all the molecules [3]. In this work a new microscopic picture is presented giving rise to the same macroscopic properties. Halogenomethanes CBr2Cl2 and CBrCl3 are known to display a glass-like transition within their low-temperature monoclinic “ordered” phases where strongly restricted reorientational motions appear. This transition from the ergodic to the non-ergodic state is associated with the freezing of the exchange positions between Cl and Br atoms.The dynamics of these simple globular-shaped and rigid molecules has been explored by dielectric spectroscopy and nuclear quadrupole resonance (NQR). Information on the origin of α- and β-relaxations in the dielectric spectra is obtained via the investigation of the isomorphous phase of CCl4 by NQR. A new microscopic mechanism underlying the glassy dynamics is proposed: The four molecules in the asymmetric unit cell are dynamically non-equivalent with respect to their molecular environment and thus perform jumps with different time scales, which are identified with the two relaxation processes