Two-dimensional lead halide perovskites (2DHPs) with a general structural formula A2A'(n-1)PbnX3n+1 (A = bulky spacer, A'= Small monovalent cation, X = I, Br, Cl) have emerged as a promising class of materials for next-generation optoelectronic devices. Suppressed photoinduced halide-ion segregation (PHS) was reported in 2D mixed halide perovskites (2DMHPs) due to suppressed ion migration, and strong excitonic effects resulting from this unique layered structure. However, 2DMHPs remain a major argument in both theoretical estimation and experimental observations. Notably, all previous reports of PHS studies in MH2DHPs are based on polycrystalline thin films[1, 2], which could be an important reason for inconsistent results due to the big variations in defect density. There is a lack of observation for the formation of iodide-rich domains under photoexcitation using photoluminescence tools, which is a signature feature in 3D PHS studies [3, 4].

Here, we demonstrate that BA2PbBrxI4-x (BA = butylammonium, x=1, 2, 3) 2DMHP single crystals have highly ordered halide stacking preference with bromide and iodide exclusively occupying B and T-sites respectively in thermal equilibrium. With hyperspectral PL and absorption imaging methods, we directly reveal the formation of a new phase upon exposure to above-bandgap excitations, which can be attributed to a photo-induced halide switching process, whereas T-site iodide switches its position with B-site bromide with local PbX64- octahedral. No PHS induced I-rich domains are observed in both PL and absorption mappings in those single-crystalline 2DMHPs, which is due to such photoisomerization does not involve any multiple unit cell mass transfer. A schematic and photoluminescence imaging of this photoisomerization of 2DMHP are presented in Figure 1. We conducted temperature dependent single crystal X-ray diffraction (SCXRD) and powder XRD (PXRD) with in situ photoexcitation measurements to reveal change in lattice constant during photoisomerization. We find that the halide switching results in an expansion of in-plane lattice constant while a reduction of interlayer distance. The new photo-switched structure and its dark form are chemically inequivalent structural isomers which exhibit distinctively different physical properties. The ability to alter local halide distributions with light while not causing phase inhomogeneity could be a key to enable a new pathway for in quantum technologies, optical switching and memory applications.