The Cr2+-based compounds, A2CrX4, where A = M+ (e.g., K+, Cs+, Rb+) or RNH3+ (e.g., MeNH3+) and X = Cl-, Br-, are an underexplored family of lead-free layered metal-halide perovskites. These compounds attracted a great deal of interest in the 1970s and 1980s after their "transparent ferromagnetism" was discovered, but they have received virtually no attention since, perhaps because they are extremely unstable in air. Further investigation into their chemistry and properties is warranted.
Large crystals of the new compound, (PEA)2CrCl4 (PEA = phenethylammonium) have been synthesized via antisolvent vapor diffusion. Remarkably, these crystals are stable in air for many days. The structure of (PEA)2CrCl4 shows 2D sheets of distorted [CrCl6]4- octahedra spaced by bulky PEA bilayers. Air stability is believed to come from this PEA bulkiness. These crystals can be mechanically exfoliated down to single monolayers with micron lateral dimensions. Magnetic measurements show in-plane ferromagnetic ordering with TC = 44 K. These materials display a unique manifestation of spin-photonic coupling: strongly forbidden Cr2+ spin-flip excitations in the visible become allowed by coupling to deexcitation of thermal "hot" magnons at zero field. A simple absorption spectrum thus gives insights into 2D spin correlations in these materials.
