HEAT CAPACITY AND ENTROPY AT THE TEMPERATURES 5 K TO 720 K AND THERMAL EXPANSION FROM THE TEMPERATURES 298 K TO 573 K OF SYNTHETIC ENARGITE (CU3ASS4)

摘要

The heat capacity of synthetic Cu3AsS4 (enargite) was measured by quasi-adiabatic calorimetry from the temperatures 5 K to 355 K and by differential scanning calorimetry from T = 339 K to T = 720 K. Heat-capacity anomalies were observed at T = (58.5 +/- 0.5) K (Delta(trs)H(m) degrees = 1.4 . R . K; Delta(trs)S(m) degrees = 0.02 . R) and at T = (66.5 +/- 0.5) K (Delta(trs)H(m) degrees = 4.6 . R . K; Delta(trs)S(m) degrees = 0.08 . R), where R = 8.31451 J . K-1. mol(-1). The causes of the anomalies are unknown. At T = 298.15 K, C(p,m)degrees and S-m degrees(T) are (190.4 +/- 0.2) J . K-1. mol(-1) and (257.6 +/- 0.6) J . K-1. mol(-1), respectively. The superambient heat capacities are described from T = 298.15 K to T = 944 K by the least-squares regression equation: C(p,m)degrees/(J . K-1. mol(-1)) = (196.7 +/- 1.2) + (0.0499 +/- 0.0016).(T/K) - (1918 000 +/- 84 000).(T/K)(-2). The thermal expansion of synthetic enargite was measured from T = 298.15 K to T = 573 K by powder X-ray diffraction. The thermal expansion of the unit-cell volume (Z = 2) is described from T = 298.15 K to T = 573 K by the least-squares equation: V/pm(3) = 10(6) .(288.2 +/- 0.2) + 10(4) .(1.49 +/- 0.04).(T/K). (C) 1996 Academic Press Limited [References: 9]