Intra-Layer Coupling in Mixed-Valence Mosaic Alloys of Layered Halide Perovskites

Layered (two-dimensional) halide perovskites present a rich structure-property space for the design of advanced electronic materials. Indeed, exploiting the molecular diversity in organoammonium cations—and myriad stable metal-halide frameworks—led to a systematic understanding of the confinement effects that distinguish these crystals from their three-dimensional congeners. Recently, our group discovered a new class of layered halide perovskites termed “mosaic” perovskites, which form through alloying of single and double layered perovskites. These mosaic alloys host three distinct metal ions at the putative <i>B</i>-site (e.g., in <i>A</i>₂<i>B</i>X₄; X = halide), notably offering an uncommon path toward mixed valency within the metal-halide framework. The first example of these disordered alloys comprised Cu(I), Cu(II), and In(III), leading to emergent optoelectronic properties rationalized by intervalence charge transfer between neighboring Cu ions.<br/><br/>Here, we build on these design principles to expand the family of mosaic layered perovskite alloys. We demonstrate that features of the known Cu(I)-Cu(II)-In(III) mosaic alloy—including the proposed role of the Cu(II) Jahn-Teller distortion and the intrinsic topology of the disordered metal-halide framework—can be used to evaluate candidates and synthesize new examples of mosaic perovskite alloys. The consequences of mixed valency on charge transfer pathways and magnetic exchange interactions will be discussed. Overall, these mosaic halide-perovskite alloys represent a platform for rational design of intra-layer coupling interactions within layered halides beyond the conventional compositional limitations of single or double perovskites. We discuss approaches to exploit such interactions alongside our perspective on the interplay between stability and disorder in these complex mixtures.