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Single-crystal layered perovskite heterostructures provide a scalable platform for potentially realizing emergent properties recently seen in mechanically stacked monolayers. We report two new layered perovskite heterostructures M2(PbCl2)(AMCHC)2(PbCl4)·2H2O (1_M where M = Na+, Li+; AMCHC = +NH3CH2C6H10COO‒) crystallizing in the chiral, polar space group C2. The heterostructures exhibit alternating layers of a lead-chloride perovskite and an intergrowth comprising corner-sharing PbCl4(η2-COO)2 polyhedra with bridging equatorial chlorides and terminal axial oxygen ligands. Small alkali metal cations and water molecules occupy the cavities between the polyhedra in the intergrowth layer. The heterostructures display wide bandgaps and two closely spaced excitonic features in their optical spectra and strong second harmonic generation. The calculated band structure of 1_Na features a Type-I quantum-well structure, where the electron-hole correlation function corresponding to the lowest excited state points to electron-hole pairs localized within a single inorganic layer (intralayer excitons), as seen in typical layered halide perovskites. In contrast, calculations show that 1_Li adopts a Type II quantum-well structure, with electrons and holes in the lowest-excited state residing in different inorganic layers (interlayer excitons). Calculations on model complexes suggest that these changes in band alignment, between Type-I and Type-II quantum-well structures, are driven by the placement of the alkali metal and the orientation of the water molecules changing the electrostatic potential-energy profiles of the heterostructures. Thus, this study sets the stage for accessing different alignments of the perovskite and intergrowth bands in bulk perovskite heterostructures that self-assemble in solution. 

The halide perovskite heterostructure (CuCl4)2(MTPA)4Cu3Cl6 (Cu_Cu; MTPA = 3-(methylthio)-propylammonium) forms from solution as single crystals consisting of alternating layers of 2D CuII–Cl perovskite and 1D CuII–Cl diamond–chain intergrowth. Using magnetometry, heat capacity, and electron paramagnetic resonance measurements, we interrogate the magnetic ordering of the 2D perovskite and 1D intergrowth layers at temperatures down to 0.055 K. As with other Cu‒Cl perovskites, the perovskite-layer spins order ferromagnetically at 10 K. Magnetization data of Cu_Cu feature a multi–component curve, consistent with magnetization of the perovskite layers and one of the three additional CuII sites in the intergrowth layer, suggesting antiferromagnetic coupling of the remaining two intergrowth-layer spins. A broad feature in AC susceptibility measurements at 6 K and an anomalous heat capacity feature at 0.3 K suggest that local ordering events occur at dramatically different energy scales with decreasing temperature. EPR spectra indicate that these local orderings occur within the 1D chains. Notably, no long–range magnetic ordering event in the intergrowth is evident down to 0.055 K, suggesting that the geometric constraints imposed by the perovskite framework and the steric bulk of the MTPA ligands physically separate and magnetically isolate the diamond chains. In contrast, well–studied diamond-spin-chain materials such as azurite show long-range magnetic order at low-temperatures due to interchain interactions. Thus, Cu_Cu provides an ideal platform for studying isolated, anisotropic spin chains. More generally, this study illustrates the capability of halide perovskite heterostructures to serve as vehicles for the scalable synthesis of complex magnetic materials. 

Addition of bromide to 2D lead-chloride perovskites appears to be a general and reliable strategy for obtaining white light at room temperature from 2D perovskites, regardless of templating effects of the organic cations.