Chemical Bonding in Covellite and Digenite: Studies By Raman Spectroscopy and XPS

Digenite (Cu1.8S) is an important non-stoichiometric copper sulphide. It has potential applications in the fields of electronics, electrocatalysis and energy conversion. However, the close compositional similarity of neighbouring sulphides makes the synthesis of monophasic digenite challenging. Also, the prevalence of the different intergrown copper sulphide phases hinders its unambiguous structural and spectroscopic examination. The paper investigates digenite formation on the inert atmosphere heat-treatment (393-673 K) of synthetic covellite (CuS) and elucidates the structural and bonding properties of both phases. The structural analysis is performed by X-ray diffraction (XRD), while the bonding characteristics are probed by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Digenite is formed at 673 K, while at lower temperatures, covellite or a biphasic mixture of covellite and anilite (Cu1.75S) is typically observed. Covellite exhibits a sharp Raman peak at 471 cm⁻¹ due to S–S stretching, whereas digenite shows a weak, broad peak at 477 cm⁻¹ due to S–S covalency. Covellite and digenite produce nearly identical photoelectron spectra. The binding energies of 2p3/2 electrons in covellite and digenite are 932.5 and 932.1 eV, respectively. Copper exists predominantly in the +1 oxidation state in both phases, with any Cu²⁺ attributable to surface contamination. Also, the two compounds, as evidenced by S(2p) electron spectra, contain mono- and also di-sulphide bonds. The study does not support the prevalence of any sulphur dangling bonds, as frequently cited in literature, on the surfaces of the sulphides.