Mosaic CuI-CuII-InIII 2D Perovskites: Pressure-Dependence of the Intervalence Charge Transfer and a Mechanochemical Alloying Method

The perovskite (BA)(4)[Cu-II(Cu(II)n(III))(0.5)]Cl-8 (1BA; BA(+) = butylammonium) allows us to study the high-pressure structural, optical, and transport properties of a mixed-valence 2D perovskite. Compressing 1(BA) reduces the onset energy of Cu-I/II intervalence charge transfer from 1.2 eV at ambient pressure to 0.2 eV at 21 GPa. The electronic conductivity of 1(BA) increases by 4 orders of magnitude upon compression to 20 GPa, when the activation energy for conduction decreases to 0.16 eV. In contrast, Cu-II perovskites achieve similar conductivity at approximate to 50 GPa. The solution-state synthesis of these perovskites is complicated, with more undesirable side products likely from the precursor mixtures containing three different metal ions. To circumvent this problem, we demonstrate an efficient mechanochemical synthesis to expand this family of halide perovskites with complex composition by simply pulverizing together powders of 2D Cu-II single perovskites and (CuInIII)-In-I double perovskites.