FIRST-PRINCIPLES THEORY OF VIBRATIONAL EFFECTS ON THE PHASE-STABILITYOF CU-AU COMPOUNDS AND ALLOYS

The importance of vibrational effects on the phase stability of Cu-Au alloys is investigated via a combination of first-principles linear re sponse calculations and a statistical mechanics cluster expansion meth od. We find that (i) the logarithmic average of the phonon density of states in ordered compounds is lower than in the pure constituents, th us leading to positive vibrational entropies of formation and to negat ive free energies of formation, stabilizing the compounds and alloys w ith respect to the phase separated state. (ii) The vibrational free en ergy is lower in the configurationally random alloy than in ordered gr ound states, which leads to lower order-disorder transition temperatur es. (iii) The random alloys have larger thermal expansion coefficients than ordered ground states, and therefore the vibrational entropy dif ference between the random and ordered states is a strongly increasing function of temperature. However, (iv) due to the associated increase in the static internal energy, the effect of thermal expansion on the free energy (and thus on the phase diagram) is only half that of the entropy alone.