A Mosaic Layered Halide-Perovskite Spin Glass: Mechanochemical Alloying of a Ferromagnet and a Paramagnet

Pulverizing together a Cr^II perovskite, (BA)₂Cr^IICl₄, and a Cr^III double perovskite, (BA)₄Ag^ICr^IIICl₈, at room temperature affords a new layered perovskite alloy incorporating three different metal ions in each layer: (BA)₈(Ag^ICr^III)-Cr^II ₂Cl₁₆ (BA = n-butylammonium). The magnetic ground state of this alloy is a spin glass (freezing temperature ∼ 3 K), which we propose arises from intrinsic disorder of superexchange interactions and a propensity to form ferromagnetic clusters. To explore the composition space beyond this example, we model the geometrical and topological properties of these complex alloys by representing the [MCl₆] ^n- tiling as effectively hard-rhombus packings on a square lattice. The structures provided by our computationally efficient model provide insight into magnetic exchange, ordering across various length scales, and the role of the in-plane Jahn-Teller distortion of the Cr^II centers in dictating the packing within the inorganic layer. By quantifying local compositional fluctuations, we identify alloy compositions at which the mixing characteristics are appreciably different from those observed in random square-lattice mixtures of three different metals. These results demonstrate a general, mechanochemical route to two-dimensional spin glasses and provide design principles and computational tools for expanding the phase space of complex layered halide perovskites with nontrivial magnetic ground states.