Low-Bandgap Cs₄ CuSb₂ Cl₁₂ Layered Double Perovskite: Synthesis, Reversible Thermal Changes, and Magnetic Interaction

Double perovskites (DPs) with a generic formula A₂ M'(I)M^III X₆ (A and M are metal ions, and X=Cl, Br, I) are now being explored as potential alternatives to Pb-halide perovskites for solar cells and other optoelectronic applications. However, these DPs typically suffer from wide (≈3 eV) and/or indirect band gaps. In 2017, a new structural variety, namely layered halide DP Cs₄ CuSb₂ Cl₁₂ (CCSC) with bivalent Cu^II ion in the place of M'(I) was reported, which exhibit a band gap of approximately 1 eV. Here, we report a mechanochemical synthesis of CCSC, its thermal and chemical stability, and magnetic response of Cu^II d⁹ electrons controlling the optoelectronic properties. A simple grinding of precursor salts at ambient conditions provides a stable and scalable product. CCSC is stable in water/acetone solvent mixtures (≈30 % water) and many other polar solvents unlike Pb-halide perovskites. It decomposes to Cs₃ Sb₂ Cl₉ , Cs₂ CuCl₄ , and SbCl₃ at 210 °C, but the reaction can be reversed back to produce CCSC at lower temperatures and high humidity. A long-range magnetic ordering is observed in CCSC even at room temperature. The role of such magnetic ordering in controlling the dispersion of the conduction band, and therefore, controlling the electronic and optoelectronic properties of CCSC has been discussed.