Vibrational spectra in ordered and disordered Ni3Al

We calculate the vibrational density of states (DOS) and corresponding thermodynamic properties of L1(2) ordered and disordered Ni3Al in the quasiharmonic approximation using the embedded-atom method. Vibrational and thermodynamic properties, including vibrational entropy differences between ordered and disordered stales, 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(2) phase, not a one-atom per cell fee 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.