The electronic structures of Cu2S and CuS have been under intense

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. 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 1 Introduction Copper sulfides are economically important ores that have found widespread use in various technologies including solar cell devies nonlinear optical material lithium ion batteries nanometer-scale switches and gas sensors.1-4 They vary widely in composition (CuxSy) and are also present as non-stoichiometric compounds. CuS and Cu2S can be considered as end members of the stoichiometric copper sulfide family.1 5 Despite their simple chemical formula both Cu2S (chalcocite) and CuS (covellite) have complex structures and several Celgosivir experimental and theoretical studies have attempted to understand their electronics and bonding.6-13 CuS has a hexagonal crystalline structure consisting of alternating layers of Rabbit Polyclonal to FAF1. planar CuS3 triangles and CuS4 tetrahedra. While CuS is a stable composition Celgosivir Cu2S is unstable towards the formation of Cu vacancies even in thermodynamic equilibrium with bulk Cu metal. The inherent instability of Cu2S and high mobility of its Cu centers has been exploited for controlled removal of Cu from Cu2S and for the generation of the known stoichiometries in the Cu-S system.14 Room temperature Cu2S is monoclinic with a complex structure containing 96 copper atoms in a unit cell.15 The crystallographic characterization of intermediate Cu2-xS (between Cu2S and CuS) systems has been difficult due to the positions of the copper atoms within the close-packed sub lattice of S atoms which are not well-defined. Interestingly important transitions in properties are observed depending on the metal to sulfur ratio in Cu2-xS systems. Cu2-xS remains diamagnetic for x = 0.0 to 0.212 although the reported magnetic behaviour of CuS differ markedly. CuS and Cu1.8S exhibit photoluminescence Celgosivir which is not observed for stoichiometric Cu2S.14 16 A large variation in electrical conductivity with Celgosivir composition has also been observed.12 Celgosivir Only CuS exhibits morphology dependent photocatalytic properties.17 Recent studies show that Cu1.8S is a good thermoelectric material.18 The presence and variation in these important properties warrants a thorough correlation of the electronic structure with the complex crystal structures of Cu2-xS systems. X-ray absorption spectroscopy (XAS) has been extensively used as a tool to determine the electronic and geometric structure of materials.19-21 However the overwhelming number of publications on hard x-ray XAS have focused on the geometric structure and only qualitative evaluation of the electronic structures has been performed. In this study we investigate the Cu and S K-edge XAS and Cu XES data using a quantitative molecular orbital (MO) theory based approach to solve the long-standing debate about their electronic structures and to correlate these with their interesting physical properties. 2 Materials and Methods 2.1 Sample Preparation Polycrystalline samples of Cu2S and CuS were prepared as previously reported.16 The samples were flame sealed in glass ampules and sent to SSRL for measurement. The ampules were transferred into a glove box and maintained under a moisture free ~1 ppm O2 atmosphere. For the Cu K-edge XAS and XES measurements polycrystalline samples were finely ground with BN into a homogeneous mixture and pressed into a 1 mm aluminum spacer between 37 μm Kapton windows. The samples were immediately frozen and stored under liquid N2. During data collection the samples were maintained at a constant temperature of ~10 K using an Oxford Instruments CF 1208 liquid helium cryostat. For S K-edge XAS studies polycrystalline samples were finely ground inside a glove box using an agate mortar and pestle and a thin layer was applied on S-free 37 μm Kapton tape placed on an aluminum frame. The samples were protected from exposure to air by a 5 μm polypropylene window placed over the front of the aluminum frame over the sample..