Synthesis, Structural and Magnetic Properties of the Solid Solution (CuCl1-xBrx)LaNb2O7 ($0 \leq x \leq 1$)

The two-dimensional (2D) quantum spin system (CuCl)LaNb2O7 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 $T_{\text{N}} = 32$ K. Here, we report on the synthesis of solid solution (CuCl1-xBrx)LaNb2O7 ($0 \leq x \leq 1$), 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 $T_{\text{N}} = 7$ K, consistent with recent $\mu$SR results, and the magnetic structure is collinear, having a significantly reduced moment. Further Br substitution leads to a linear increase in $T_{\text{N}}$ up to $x = 1$. These results indicate that (CuCl)LaNb2O7 is located in the vicinity of the quantum phase boundary.