Using bond-length dependent transferable force constants to predict vibrational entropies in Au-Cu, Au-Pd, and Cu-Pd alloys

A model is tested to rapidly evaluate the vibrational properties of alloys with site disorder. It is shown that length-dependent transferable force constants exist, and can be used to accurately predict the vibrational entropy of substitutionally ordered and disordered structures in Au-Cu, Au-Pd, and Cu-Pd. For each relevant force constant, a length- dependent function is determined and fitted to force constants obtained from first-principles pseudopotential calculations. We show that these transferable force constants can accurately predict vibrational entropies of L1${}_2$-ordered and disordered phases in Cu${}_3$Au, Au${}_3$Pd, Pd${}_3$Au, Cu${}_3$Pd, and Pd${}_3$Au. In addition, we calculate the vibrational entropy difference between L1${}_2$-ordered and disordered phases of Au${}_3$Cu and Cu${}_3$Pt.