Structural Modulation and Spin-Singlet Ground State Found in Antiferromagnets Bi <i>A</i> O ₂ Cu ₂ (SO ₄ ) ₂ ( <i>A</i> = K, Rb, Cs)

A new class of quasi-one-dimensional quantum magnetic materials, BiAO2Cu2(SO4)2 (A = K, Rb, Cs), were successfully synthesized via a conventional hydrothermal method. Systematic characterization of their structural and magnetic properties reveals that the radius of alkali metal cation A+ plays a critical role in structural tuning. Structurally, the K and Rb compounds are isostructural, crystallizing in the monoclinic system (P21/m). The Cu1O5 square pyramids and Cu2O4 square planes form a uniform chain via μ2-O bridges. In contrast, substitution with the larger Cs+ ions at the A-site reduces the crystal symmetry to the triclinic system (P-1). Specifically, the pronounced steric effect induced by the Cs+ ions causes a relative rotation of the SO4 tetrahedra. This rotation consequently modifies the coordination geometry of the Cu2+ ions, resulting in a five-coordinate, distorted square-pyramidal configuration for each copper site and thereby transforming the copper chain into an alternating chain. Magnetic measurements indicate a nonmagnetic spin-singlet ground state for all compounds. The formation of this spin-singlet ground state may originate from the presence of significant antiferromagnetic next-nearest-neighbor interactions along the chain direction.