ABSTRACT
In spite of a simple chemical formula, the phase diagrams and crystal structures of Cu2−δX (X = S, Se, and Te) are quite complex, and they depend sensitively on the amount of Cu deciency δ. For example, besides the stoichiometric Cu2S (chalcocite), there exist several other kinds of Cu2−δS compounds, termed Cu1.93S-Cu1.96S (djurleite), Cu1.8S (digenite), Cu1.75S (anilite), and CuS (covellite), which are experimentally proven as stable structures at room temperature [33]. After at least one phase transition, the room temperature Cu2−δX phase gradually converts to the nal high-temperature cubic phase, with the actual temperature and number of phase transitions determined by δ. The high-temperature Cu2−δX cubic phases are stable over a wide composition range, for example, δ = 0-0.25 for Cu2−δSe34 and δ = 0-0.27 for Cu2−δS [33]. Now, the crystal structures of the high-temperature Cu2−δX cubic phase have been already clearly determined, while the crystal structures of their lowtemperature phases have not been completely claried yet. Specically, the Cu deciency would destroy the initial lattice periodicity in stoichiometric Cu2X phases and induce more complex atomic arrangements, enhancing the difculty and uncertainty when determining the crystal structure of low-temperature Cu2−δX phase. Thus, here we only discuss the phase diagrams and crystal structures of the stoichiometric Cu2X (X = S, Se, and Te) compounds.
