Vibrational dynamics in the undercooled liquid of ultra-fragile metallic glasses

Abstract

The lattice vibrational dynamics in ultra-fragile Pt₅₇Cu₂₃P₂₀ and Pt₆₀Cu₂₀P₂₀ glasses were investigated using in situ inelastic neutron scattering to measure the evolution of the phonon density of states (DOS) through the glass transition, into the undercooled liquid phase, and through crystallization. The phonon DOS curves are remarkably similar for the undercooled liquids and the amorphous solids. The vibrational entropy was obtained with these phonon DOS curves, and changes in vibrational entropy caused by the glass transition are less than 0.05 k_B/atom. For these highly fragile metal/metalloid glasses, the excess vibrational entropy of the undercooled liquid is about 0.5–1.5 J mol⁻¹ K⁻¹, compared to 0.1 J mol⁻¹ K⁻¹ reported earlier for stronger Cu–Zr glasses. Our work suggests that, consistent with our previous report for less fragile metallic glasses, the fraction of the total excess entropy that is vibrational is small, and the discontinuity in excess entropy at the glass transition is mainly due to configurational entropy.

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