Vibrational entropy of L1(2) Cu3Au measured by inelastic neutron scattering

The phonon density of states of elemental Au, Cu, and Cu3Au with L1(2) chem ical order were measured by inelastic neutron scattering and used to calcul ate the vibrational entropy of formation of the ordered compound from the e lemental metals. A vibrational entropy of formation of (0.06 +/- 0.03) k(B) /atom at 300 K was obtained, with the vibrational entropy of the ordered al loy being larger than that of the elemental metals. The phonon DOS of the d isordered Cu3Au was simulated by adding the phonon DOS curves of fee Cu, L1 (2) Cu3Au, and fee Au to match the numbers of first-nearest-neighbor pairs in a disordered alloy. The vibrational entropy obtained with this simulated DOS disagrees with calorimetric data and theoretical estimates, indicating that the phonon DOS of disordered Cu,Au depends on chemical order at spati al lengths larger than is set by first-nearest-neighbor pairs. [S0163-1829( 99)02429-7].