Dynamics of Restricted Reorienational Motions in Long-range Ordered Lattices

MARIANO ZURIAGA; LUIS CARLOS PARDO; PETER LUNKENHEIMER; JOSEP LLUIS TAMARIT; NESTOR VEGLIO; MARIA BARRIO; FRANCISCO JAVIER BERMEJO; ALOIS LOIDL

Foz do Iguazu, Brasil

Conferencia; International Conference on the Physics of Non Crystalline Solids XII; 2009

Dielectric spectra of glassy materials display a main relaxation mode, directly linked to the glassy freezing of molecular motion, and a secondary relaxation whose microscopic origin is not clear at all. This secondary relaxation, called Johari-Goldstein β (JG-β) relaxation, is commonly regarded as a universal feature of glassy matter whose understanding is prerequisite to achieve a better knowledge of the glass transition and the glassy state of matter. In the present communication, we report dielectric spectra for a very simple type of glass formers where only very restricted motions of small, rigid molecules are possible. The materials in question are some halogenomethanes CBrnCl4-n, n =0, 1, 2. They exhibit, in addition to a series of thermally induced solid-solid phase transitions, a low-temperature monoclinic (C2/c) structure with Z=32 molecules per unit cell and with 4 molecules per asymmetric unit. Thermodynamic measurements in this phase showed an anomaly at ca. 90 K that displays all the characteristics of a glass-transition and it involves the freezing of orientatonal degrees of freedom. The dynamics of these simple glassy systems has been investigated by means of the concurrent use of dielectric and nuclear quadrupole resonance spectroscopy (NQR) measurements, providing informationabout time correlations concerning macroscopic polarization and microscopic dynamics as sensed by Cl nuclei, respectively.