The difference between the heat capacities of chemically disordered and Lh-ordered Q13AU was measured by calorimetry from 70 to 300 K. By comparing these measured results with a harmonic heat capacity calculated with a Born-von Karman model, we estimate the difference in vibrational entropy between disordered and ordered CU3AU to be (0-14 ± 0'05)A:B atom""' at high temperatures. The smaller vibrational entropy of the ordered alloy originates from features of its optical modes, features which seem related to the different sizes of Cu and Au atoms.
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