Dynamics in a Nonfragile Glass-Forming Liquid
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G. Coddens, J.P. Ambroise L.aborttoire Lvon Brilkouin, CE Saclay, 91191 Gif Sur Yvette, France
E. Gu6guen. R. Marchand G;roipe Verres el Ccrarniques,URA CNRS 1496, Universitg de Rennes 1,35042 Rennes, France
ABSTRACT The dynamics of Na 0 .5 Li 0 .5 PO 3 (Tg = 515 K, Tm = 749 K) a non fragile glass forming liquid has been investigated over a large temperature range (300 - 1000 K) and in a wide energy window Lising various experimental techniques. The susceptibility spectra obtained by coherent neutron scattering and depolarized light scattering between I and 104 GHz show mainly two contributions: a low frequency vibrational peak, the so-called Boson peak and a quasielastic component. referred to the Pfast process in the mode coupling theory (MCT). The data are discussed in relation to the mode coupling theory for the liquid glass transition. In particular, the temperature evolution of the susceptibility height in the pfast region is compatible with a crossover temperature Tc - 620 K which is also deduced from a power law temperature dependence of the structural relaxation timescale. As a secondary P3slow process, observed by 3 1P NMR, decouples from the structural relaxation timescale also below 600 K, a real change in the dynamics seems to occur around Tc - 620 K = 1.2 Tg in this non fragile glass forming liquid. INTRODUCTION During the last years, the mode coupling theory [1] has been shown to successfully describe neutron and light scattering data [2-9] for the dynamics of some weakly supercooled liquids which are called fragile [10]. Indeed, a two step (J0fast and (x ) relaxation behavior for the correlation function of the density fluctuations is observed and a crossover temperature Tc > Tg is identified as predicted by the MCT. In the idealized version of the theory, an ergodic-nonergodic transition occurs at Tc where the density fluctuation modes arrest with decreasing temperature. In an extended version, temperature activated hopping processes have been taken into account to restore ergodicity below Tc as experimentally observed. However, it seems that in less fragile systems, the MCT fails to describe the dynamics at high frequencies. It has been related to the stronger vibrational contribution, present in these glass formers. Indeed, it is now well known that the Boson peak contribution to the dynamical structure factor S(q.w)) strongly increases when going from fragile towards strong glass forming liquids [1 11. Thus in non fragile systems, this inelastic contribution strongly distorts the dynamical susceptibility above the susceptibility minimum [12-15] between the czand Pfast processes which is just the frequency range concerned by some predicted scaling laws of the MCT. The Boson peak contribution could thus be responsible for the departure of the asymptotic exponent from the constraint imposed by the MCT, as already slightly observed in fragile liquids and strongly amplified in less fragile systems [16]. EXPERIMENTAL RESULTS Nao.5LiO.5PO3 is the eutectic composition (Tg = 515 K, Tm = 749 K) based on
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