Thermodynamics of disordering in Au3Cu

Abstract

The enthalpy of disordering, as well as the vibrational and the configurational entropy of disordering of stoichiometric Au3Cu were investigated by heat capacity measurements using differential scanning and relaxation calorimetry. Beside these calorimetric methods, the density functional theory was used to calculate the enthalpy of disordering at zero K. The time scale of the ordering reaction was larger than that of the calorimetric experiment resulting in non-equilibrated heat capacity data. Integrating these data within appropriate limits delivered, however, equilibrium values for the enthalpy of disordering, from which the heat capacity and the entropy of disordering were calculated. Investigating the low-temperature heat capacity behaviour of ordered and disordered samples enabled the separation of vibrational and configurational parts of the entropy. The enthalpy of disordering in Au3Cu is lower than half of that in Cu3Au and reaches a maximum value of 1.46 kJ mol−1. The entropy of disordering at 650 K has a value of 0.58 R with a small vibrational part of 0.02 R. The determined configurational entropy reaches the ideal configurational entropy of a disordered A3B alloy of 0.562 R at 650 K. Above 650 K, the ideal value is slightly exceeded.