Determination of the Infinite Frequency Sound Velocity in the Glass Former o -Terphenyl

The temperature dependence of the acoustic dispersion relation, Ω(Q,T), has been determined across the liquid-glass transition in o-terphenyl using inelastic x-ray scattering in the 1--6nm^-1 momentum transfer ( Q) range. From this the infinite frequency sound velocity v_∞(T) can be obtained. In the glass, v_∞(T) compares well with existing light scattering determinations, while in the liquid its value can be used to discriminate among different phenomenological models previously proposed to infer v_∞(T) from low temperature data. Moreover, the Debye-Waller factors ( fQ) in two different Q regimes ( Q≈0 and Q0ex0ex=0ex0ex2.5nm^-1) are determined and found to be mutually consistent.