Specific heat of copper-gold alloys below 30 K

Abstract

Specific-heat measurements in the 0.4- to 30-K range on both ordered and disordered alloys containing 25, 50, and 75 at.% Au are reported and analyzed into nuclear, electronic, and lattice components. The disordered alloys containing 25 and 50 at.% Au are unstable at room temperature and slowly order with specific-heat "half-lives" of about 2000 and 100 days, respectively. No instablility was observed for the 75-at.% Au alloy which has a lower order/disorder transition temperature (∼ 200 °C) than the others (∼ 400 °C). Order apparently improved in the long-period superlattice CuAu II when held at room temperature although CuAu I is the stable phase here. Results on Cu${\mathrm{Au}}_3$ are consistent with the existence of a Cu${\mathrm{Au}}_3$ II (long-period superlattice) ordered phase as well as the simple ordered structure Cu${\mathrm{Au}}_3$ I. The above results suggest that antiphase domain boundaries cannot move at room temperature although long-range order can improve here. Also, that both random and periodic antiphase domain structures have similar effects on the specific heat. The lattice specific heats of the ordered phases CuAu I and Cu${\mathrm{Au}}_3$ I were anomalous, both showing an initial increase (∼ 10 and ∼ 5%, respectively) in Debye temperature as temperature is increased from zero. For CuAu I this could be a two-dimensional effect related to the layered structure. A similar explanation may be attempted for the case of Cu${\mathrm{Au}}_3$ I.