Anion Exchange and\nLateral Heterostructure Formation\nin Ferromagnetic PEA<sub>2</sub>Cr(Cl,Br)<sub>4</sub> Two-Dimensional\nPerovskites

Postsynthetic vapor-phase anion exchange in the ferromagnetic\ntwo-dimensional\n(2D) hybrid metal-halide perovskite, PEA<sub>2</sub>CrCl<sub>4</sub> (PEA<sup>+</sup> = phenethylammonium), is reported. Anion exchange\nusing vaporous trimethylsilyl bromide (TMS-Br) is shown to drive complete\nconversion of solution-processed PEA<sub>2</sub>CrCl<sub>4</sub> polycrystalline\nthin films to PEA<sub>2</sub>CrBr<sub>4</sub>. Low-temperature magnetic\ncircular dichroism spectroscopy indicates ferromagnetic ordering in\nthese PEA<sub>2</sub>CrCl<sub>4</sub> and PEA<sub>2</sub>CrBr<sub>4</sub> films. Via partial anion exchange of exfoliated flakes of\nPEA<sub>2</sub>CrCl<sub>4</sub> single crystals, we demonstrate that\nit is possible to generate abrupt lateral PEA<sub>2</sub>CrCl<sub>4</sub>/PEA<sub>2</sub>CrBr<sub>4</sub> magneto-heterointerfaces.\nKinetic studies reveal that lateral heterostructure formation is dictated\nby rapid edge-site halide exchange followed by slower intralayer bromide\ndiffusion, and there is negligible interlayer (3D) bromide or TMS-Br\ndiffusion. The importance of the bulky PEA<sup>+</sup> interlayer\ncation in suppressing 3D diffusion is highlighted by parallel anion-exchange\nexperiments on MA<sub>2</sub>CrCl<sub>4</sub> (MA<sup>+</sup> = methylammonium),\nwhich instead show 3D exchange. Comparison of anion-exchange reactions\nin PEA<sub>2</sub>CrCl<sub>4</sub>, PEA<sub>2</sub>MnCl<sub>4</sub>, and PEA<sub>2</sub>PbCl<sub>4</sub> shows that 2D bromide diffusion\nis slowest in PEA<sub>2</sub>CrCl<sub>4</sub>, attributed to the antiferrodistortive\nordering found in this composition. In addition to demonstrating both\npostsynthetic composition control and heterostructure formation in\nferromagnetic Cr-based 2D perovskites for the first time, these results\nalso advance our fundamental understanding of ion-exchange processes\nin this relatively unexplored family of 2D perovskites, broadening\nopportunities for investigation and control of novel spin effects\nin low-dimensional metal-halide perovskites.