Heteroanionic 2D Ruddlesden-Popper Perovskites A₂CdI₂Cl₂ (A = Cs, MA)— Synthesis, Structure and Optical Characterization

Halide perovskites are next-generation photovoltaic candidates due to their remarkable properties, such as defect tolerance, low thermal conductivity, and efficient light emission, which arise in part from the structural dynamics and disorder of the crystal lattice. However, the limits of this disorder are unclear – that is, how far can the perovskite structure be stretched while maintaining similar properties? The 2D Ruddlesden-Popper perovskites (RPs), A_n+1B_nX_3n+1, are quasi-2D <100> structure variants of 3D perovskites where B⁺ ion occupies in corner-connected octahedra in ab plane, while A⁺ ions space out the layers. Here, we study inorganic and hybrid mixed-anion 2D RP materials to understand the structure-property relationship and gain insights into the origin of their optical properties. In this presentation, we report the synthesis, crystal structure, and optical properties of two novel RPs, Cs₂CdI₂Cl₂ and MA₂CdI₂Cl₂ (MA = methylammonium).

All-inorganic Cs₂CdI₂Cl₂ crystals were grown using a solvothermal reaction in a 1:1 HCl and HI mixture, while the hybrid MA₂CdI₂Cl₂ was grown using a hydrothermal reaction in distilled water with a few drops of hypophosphorous acid and followed by slow evaporation crystallization. Powder X-ray diffraction of Cs₂CdI₂Cl₂ and Rietveld refinement confirm that the material crystallizes in the tetragonal I4/mmm space group. This is the highest allowable symmetry for Ruddlesden-Popper perovskites, and is isostructural with Rb₂CdI₂Cl₂, Cs₂SnI₂Cl₂, and Cs₂PbI₂Cl₂ analogues. This series is thus ideal for probing the impact of A and B site variation in these mixed-anion compounds, which host heteroleptic coordination of the B site with both I and Cl ions. On the other hand, MA₂CdI₂Cl₂ crystallizes in the orthorhombic Fmm2 space group, confirmed by single crystal X-ray diffraction. The octahedral units of this structure are tilted due to the orientation of the MA cation in the void, causing the reduction in symmetry. Photoluminescence of Cs₂CdI₂Cl₂ shows a broad yellow emission centered at 570 nm with 340 nm excitation. Comparison of these new 2D RPs with the known compounds Rb₂CdI₂Cl₂ and Cs₂PbI₂Cl₂, which have exhibited broadband emission and photoconductivity, respectively, will enable us to connect structural features, including the presence or absence of a lone pair, with optical properties to close the structure-property relationship loop. In this presentation, we will discuss the synthesis and crystal growth of these materials, their crystal structures, and their optical properties measured by photoluminescence and UV-Vis absorption spectra.