Vibrational spectra in ordered and disordered Ni3Al

We calculate the vibrational density of states (DOS) and corresponding thermodynamic properties of L1{sub 2} ordered and disordered Ni{sub 3}Al in the quasiharmonic approximation using the embedded-atom method. Vibrational and thermodynamic properties, including vibrational entropy differences between ordered and disordered states, are found to be in good agreement with experiment. The DOS of the configurationally disordered alloy resembles a broadened version of the DOS of the L1{sub 2} phase, not a one-atom per cell fcc DOS, and is shifted downward in frequency because the disordered state has a larger volume than the ordered phase. Calculations of the projected DOS indicate that high-frequency modes located predominantly on aluminum atoms broaden the most on disordering. Further, we find that the vibrational entropy difference between the two phases is largely due to the difference in volumes of the phases and their different thermal expansions. {copyright} {ital 1997} {ital The American Physical Society}