Synthesis, structural and magnetic properties of the solid solution (CuCl_1-xBr_x)LaNb₂O₇ (0 < x < 1)

The two-dimensional (2D) quantum spin system (CuCl)LaNb₂O ₇ has a spin-singlet ground state with a gap of 2.3 meV, while the isostructural material (CuBr)LaNb₂O₇ displays a collinear antiferromagnetic order at T_N = 32 K. Here, we report on the synthesis of solid solution (CuCl_1-xBr_x)LaNb ₂O₇ (0 < x < 1), and its structural and magnetic properties investigated by magnetic susceptibility, high-field magnetization, and neutron diffraction measurements. The x dependences of cell parameters follow Vegard's law, verifying the uniform distribution of Cl and Br atoms at the halide site, although a more complex structural evolution is inferred from an opposing correlation between the intra- and interlayer cell distances (vs x). 5%-Br substitution is found to induce an antiferromagnetic order with T _N = 7 K, consistent with recent μSR results, and the magnetic structure is collinear, having a significantly reduced moment. Further Br substitution leads to a linear increase in T_N up to x = 1. These results indicate that (CuCl)LaNb₂O₇ is located in the vicinity of the quantum phase boundary.