Crystal structure analysis and refinement of digenite, Cu_1.8S, in the temperature range 20 to 500 °C under controlled sulfur partial pressure

Low- and high-digenite, Cu_1.8S, has been studied by X-ray diffraction at room temperature and at temperatures up to 500 °C. For the experiments the Debye-Scherrer technique was used, with buffered sulfur partial pressure at all temperatures. The crystal structures of high-digenite have been determined at 200 °C, 300 °C, 400 °C, and 500 °C, obtaining reliability values between R = 1.9 % and 1.0%. The four sulfur atoms (per unit cell) form a fcc lattice in Fm3m, while the 7.2 Cu atoms occupy statistically three sites 8c, 4b and 1921. With increasing temperature, copper atoms leave the 8c sites and migrate to the 4b sites. All Cu positions are only partially filled.

Above 436 °C high-digenite is identical with “blue chalcocite” (Buerger, 1942) with the composition Cu_1.8S crystallizing in Fm3m like the other high-digenites. The structure at 500 °C has been refined. 8c is occupied to 25% and 4b to about 6%. Below 436 °C high-chalcocite crystallizes hexagonal in space group P6₃/mmc. Data for chalcocite have been collected at 300 °C, and the crystal structure refined with R = 2.6%.