Bonding topology and electronic characterization of mechanochemically synthesized CsCuBr3 as a lead-free p-type semiconductor.

In this work, we present a systematic study of the copper halide compound CsCuBr3, combining mechanochemical synthesis with structural, spectroscopic, electrochemical, and first-principles investigations. Phase-pure orthorhombic CsCuBr3 (space group C2221) was obtained after 3 h of planetary ball milling under atmospheric conditions. Structural and microstructural evolution were carefully monitored via Rietveld refinement and Williamson-Hall analysis, while X-ray photoelectron spectroscopy confirmed the expected composition and the Cu2+ oxidation state. Diffuse reflectance spectroscopy revealed broad absorption extending from the UV to the near-infrared, consistent with charge transfer and Cu2+ d-d transitions. The optical band gap determined using the inverse logarithmic derivative method was 1.40 eV ± 0.04 eV, with a transition parameter of m = 0.49 ± 0.01, consistent with a direct-allowed transition. Spin-polarized DFT + U calculations including spin-orbit coupling yielded a narrow direct gap of 1.29 eV and a weakly antiferromagnetic lowest-energy configuration. QTAIM analysis revealed a non-trivial bonding topology within the strongly Jahn-Teller distorted [Cu2Br9]5- unit, showing that only four of the six Cu-Br contacts contain bond critical points, and these do not correspond simply to the four shortest Cu-Br distances. Electrochemical measurements yielded frontier energy levels of EVB = -5.14 eV and ECB = -3.62 eV, while Mott-Schottky analysis was consistent with p-type behavior, with an acceptor density of ∼1016 cm-3 and a flat-band potential of 0.79 V vs. Ag/Ag+. These results establish a detailed relationship between the local bonding topology and electronic structure of CsCuBr3 and highlight its potential for further investigation in lead-free optoelectronic applications.