The Emergence of Halide Layered Double Perovskites

Halide double perovskites have allowed the significant expansion of the possible metals and oxidation states in halide perovskites. Further, they have shown some remarkable properties and applications. Akin to the dimensional reduction in halide perovskites to generate layered perovskites, it has recently been shown that it is also possible to dimensionally reduce double perovskites to generate layered double perovskites (LDPs). The implications of such realization are tremendous from several different perspectives. First, it widens the space of possible metals, as LDPs can be made of BI, BII, and BIII cations and their permutations. Second, it allows the modulation of the materials' electronic structure through dimensional reduction and quantum confinement effects. Third, it allows the incorporation of many more organic cations, thereby providing a more chemically, structurally, and electronically diverse pool of materials. The combinations of these three factors result in a potentially infinite family of new materials with new or improved properties and applications. Herein, we describe the emergence of this new family and the known chemical, structural, and electronic features and review their proved and potential applications. Finally, we highlight some of the challenges and propose some future avenues of research and areas of opportunity for these materials.