DYNAMICS OF DENSITY AND ORIENTATION FLUCTUATIONS IN SUPERCOOLED ZINC HALIDES

Photon correlation spectroscopy in both polarized and depolarized geom etries has been utilized to study density and orientation fluctuations in ZnCl2, ZnBr2, and their symmetric mixture in the metastable superc ooled stare. The pure components display behavior intermediate between strong and fragile glasses reflected in the shape (L(log tau)) and dy namics (tau) of the relaxation function C(t), being similar for both d ensity (C-rho(t)) and orientation (C-2(t)) correlation functions. In c ontrast to polymer blends, concentration fluctuations have no measurab le broadening effect on the shape of C(t). Instead, L(log tau) becomes narrower with increasing temperature and affects differently C-rho(t) and C-2(t) for the zinc halide mixture. Despite local homogeneity in the thermodynamically ideal mixture, ZnBr2 with the higher glass trans ition temperature (T-g = 395 K) was found to dominate the primary alph a-relaxation in the symmetric binary mixture. In the framework of ''tw o-fluid'' models, the distribution and the temperature dependence of t au support rigid microheterogeneities on the order of 2 nm. The additi onal slow process, observed in fragile glasses so far, appears to affe ct also the anisotropic scattering from the pure components.