Quantitative X-ray absorption and emission spectroscopies: electronic structure elucidation of Cu2S and CuS

The electronic structures of Cu₂S and CuS have been under intense scrutiny, with the aim of understanding the relationship between their electronic structures and commercially important physical properties. Here, X-ray absorption and emission spectroscopic data have been analyzed using a quantitative, molecular orbital (MO) based approach to understand the electronic structure of these two complex systems. Cu₂S is shown to have a significant amount of Cu²⁺ sites and therefore Cu⁰ centers. The presence of low-valent Cu is correlated with the electrical conductivity of Cu₂S, especially at high temperatures. CuS is shown to have tetrahedral Cu²⁺ and trigonal Cu¹⁺ sites, with crystal planes that have alternating high and low charge on the Cu centers. These alternating charges may contribute to internal energy transitions required for photoluminescence properties. The in-depth electronic structure solutions presented here not only solve a complicated much-debated problem, but also demonstrate the strength of quantitative MO based approach to X-ray spectroscopies.