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].