Comparative analysis of the fast dynamics in the supercooled nonfragile glass-forming liquid Na 0.5 Li 0.5 PO 3 observed by coherent neutron scattering

Experimental results obtained by coherent neutron scattering with a time-of-flight spectrometer on the nonfragile glass-forming liquid ${\mathrm{Na}}_{0.5}{\mathrm{Li}}_{0.5}{\mathrm{PO}}_{3}$ (${T}_{g}=515\mathrm{K},$ ${T}_{m}=749\mathrm{K}$) in a wide temperature range (300\char21{}773 K) are presented. As predicted by the mode coupling theory (MCT), the resulting dynamical structure factor has a $q$-independent form in the intermediate ${\ensuremath{\beta}}_{\mathrm{fast}}$ regime. Furthermore, it is also found that the $q$ dependence of the dynamical structure factor is the same for the boson peak and the quasielastic region. Thus, a phenomenological model assuming that the quasielastic component is related to the damping of the localized vibrational modes that give rise to the boson peak is discussed. Though the spectra are well described over a wide temperature range, it is shown that the temperature ${T}^{*}$ where the boson peak becomes overdamped is significantly above the melting temperature in this system and thus cannot be associated to any onset of metastability as previously suggested. On the other hand, a qualitative discussion of the susceptibility spectra above ${T}_{g}$ in the frame of the MCT leads to a possible crossover temperature ${T}_{c}$ near 620 K $(\ensuremath{\sim}1.2{T}_{g})$ close to the temperature of the decoupling phenomenon of the relaxation time scales observed by ${}^{31}\mathrm{P}$ NMR in this system. This ${T}_{c}$ value is also compatible with a power-law temperature dependence of the structural relaxation time scale deduced from the viscosity data.