Magnetic Copper Selenite-Sulfate Cu ₃ O ₂ (HSeO ₃ )(HSO ₄ )(H ₂ O) with T26-Type Non-Archimedean Lattice Featuring a Robust 1/3-Magnetization Plateau

Discovering novel magnetic materials with unique spin-lattices for exotic quantum phenomena is a significant challenge. Here, we successfully synthesized a new antiferromagnet, Cu₃O₂(HSeO₃)(HSO₄)(H₂O), using mixed-anions of trigonal-pyramidal SeO₃²⁻ and tetrahedral SO₄²⁻. Cu₃O₂(HSeO₃)(HSO₄)(H₂O) consists of corrugated layers forming a rare T26-type non-Archimedean (2-uniform) lattice (Figure 1f), where diamond chains are interconnected through their dimers to form hexagons between spin chains. Despite a large Weiss-temperature θ of -355(1) K, Cu₃O₂(HSeO₃)(HSO₄)(H₂O) exhibits antiferromagnetic order at 2.3 K, indicating strong quantum spin fluctuations. Interestingly, high-field magnetization measurement shows a robust 1/3-magnetization plateau that remains stable up to 54 T. This phenomenon was not theoretically predicted for regular T26-type non-Archimedean lattices. Notably, Cu₃O₂(HSeO₃)(HSO₄)(H₂O) represents the first example of all reported non-Archimedean lattices featuring a robust 1/3-magnetization plateau. Our DFT calculation results reveal that the robust 1/3-magnetization plateau arises from the fact that the distorted T26-type non-Archimedean lattice of Cu₃O₂(HSeO₃)(HSO₄)(H₂O) can be further simplified as a trimer with strong intratrimer interaction of -500 K, requiring a much higher magnetic field to overcome for full saturation. Our work demonstrates that utilizing mixed anions as bridging pathways to regulate magnetic couplings is a promising approach for discovering novel spin lattices with quantum phenomena.