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

A new class of quasi-one-dimensional quantum magnetic materials, Bi A O 2 Cu 2 (SO 4 ) 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 ( P 2 1 / m ). The Cu1O 5 square pyramids and Cu2O 4 square planes form a uniform chain via μ 2 -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 SO 4 tetrahedra. This rotation consequently modifies the coordination geometry of the Cu 2+ 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.