Syntheses, Structures and Optoelectronic Properties of Copper(I) Tungstate/Molybdate Solid Solutions

Solid solutions have emerged as an effective strategy for modulation of the optoelectronic properties of semiconductors. Herein, a Cu(I)-containing molybdate, Cu₆Mo₅O₁₈, and its solid solution with tungsten, Cu_6-xMo_5-yW_yO₁₈ (y = 0.5, 1.0, 1.5, and 2.1), have been synthesized via both solid-state reactions and a rapid arc-melting approach that takes only seconds. Both methods yielded crystals that were characterized by single-crystal X-ray diffraction and consist of distorted (Mo/W)O₆ octahedra condensed via edge-sharing into zigzag chains. These chains form layers capped by additional (Mo/W)O₆ octahedra and bridged by Cu(I) cations within distorted CuO₄ tetrahedra. The W(VI) cation exhibits a site preference for coordination within the chains (moderately C₂/C₄-distorted) versus the capping position (strongly C₃-distorted), as confirmed by electronic structure calculations. Their black powders show small bandgaps which decrease from 1.25(2) to 1.12(2) eV with increasing tungsten. Calculations show optical absorption coefficients greater than 1 × 10⁵ cm^-1 at only 1 eV and steeply rising to 5 × 10⁵ cm^-1 by 2.4 eV. Their bandgaps were found to stem from valence and conduction band edges comprised of filled Cu 3d-based and empty Mo 4d-based states, respectively, with the W 5d-orbitals having negligible impact. Hence, these findings demonstrate the efficient synthesis of Cu(I)-molybdates/tungstates and their promising optoelectronic properties.