The Copper–Selenium System at Temperatures to 850 K and Pressures To 50 Kbar

Phase relationships in the copper/selenium system in the composition range 30–70 atomic % selenium have been studied at temperatures from 298 to 850 K and at pressures to 50 kbar. A revised atmospheric pressure phase diagram is given, as well as an outline of the phase diagram at 20 kbar.αCu2Se is monoclinic at 298 K with a = 14.087, b = 20.481, c = 4.145 Å, β = 90° 23′. The α → β (f.c.c.) transformation is complex, and occurs over a 30 K interval centered on 396 K. The overall enthalpy change is 6.4 ± 2 kJ mol−1. The maximum in the DTA signal for this transition decreases slowly with increasing pressure at < 1 K kbar−1. The signal disappears above 42 kbar, presumably due to the formation of a high pressure modification.Variable composition in Cu2−xSe (berzelianite) extends from Cu2.00Se to Cu1.75Se in the range 402–523 K. Cu1.80Se, with a = 5.765 Å at 298 K, is stable throughout the temperature and pressure ranges investigated.Cu3Se2 (umangite) is stable at 298 K to at least 35 kbar. Dimensions of the tetragonal cell at 298 are a = 6.385, c = 4.217 Å. At atmospheric pressure it disproportionates to Cu2−xSe and βCuSe at 386 K with an enthalpy change of 10.0 ± 5 kJ mol−1. The reverse reaction is very slow. Above ca. 5 kbar it disproportionates to Cu2−xSe and CuSe2II at a temperature less than 413 K. The reverse reaction is fast.αCuSe (klockmannite) is hexagonal with a = 3.934, c = 17.217 Å at 298 K. It transforms to βCuSe at 323 K under atmospheric pressure with an enthalpy change of 0.84 ± 5 kJ mol−1. This modification is C end-centered orthorhombic with a = 3.948, b = 6.958, c = 17.239 Å at 324 K. With increasing temperature the orthorhombic a/b ratio increases rapidly until, at 393 K, the transition to γCuSe is complete. This modification is hexagonal with a = 3.984, c = 17.288 Å at 430 K. CuSe is unstable above 5 kbar at 298 K, decomposing to form Cu3Se2 and CuSe2II.Marcasite-type CuSe2, with a = 5.0046, b = 6.1822, c = 3.7397 Å at 298 K, disproportionates to CuSe and Se at 605 K under atmospheric pressure with an enthalpy change of 9.6 ± 4 kJ mol−1. It transforms at less than 5 kbar at 298 K to the cubic pyrite modification CuSe2II, with a = 6.116 Å. CuSe2II melts congruently at 818 K and 20 kbar. The melting point increases with increasing pressure at ca. 3 K kbar−1.