Optical properties and energy band scheme of cuprous sulphides with ordered and disordered copper ions

In previous works the optical properties of thin single crystals of chalcosite (Cu2.00S) and the copper-deficient phases djurleite (Cu1.96S), Cu1.9S and digenite (Cu1.8S) are reported. Here the effects of the order—disorder transition of the copper sublattice in chalecosite at 105°C on the optical properties of chalcosite are investigated. Previous and present data lead to the proposal of the following energy band scheme for the cuprous sulphides. 1. The valence band edge of chalcosite is at a non-zero value of the electron wave vector. 2. Djurleite can be considered, in a rigid-band model, as a form of chalcosite with a partly depleted valence band, where the depletion is restricted to the “indirect” maximum. 3. This “indirect” maximum is essentially unchanged by the order—disorder transition of the copper ions in chalcosite at 105°C and therefore seems to be associated primarily with sulphur orbitals. This picture leads to a reasonable estimate for the effective mass of holes in chalcosite and offers an explanation for the existence of two very similar varieties of djurleite existing in equilibrium with orthorhombic and supercooled hexagonal chalcosite, respectively.