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Synthesis, Structural and magnetic properties of the solid solution (CuCl1-xBrx)LaNb2O7 (0<x<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 collinear antiferromagnetic order at TN = 32 K. Here we report on the synthesis of solid solution (CuCl1-xBrx)LaNb2O7 (0 < x < 1), and its structural and magnetic properties by means of magnetic susceptibility, high-field magnetization, and neutron diffraction studies. The x-dependence of cell parameters follows Vegards law, verifying the uniform distribution of Cl and Br atoms at the halide site, though 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 antiferromagnetic order with TN = 7 K, consistent with a recent mSR study, and the magnetic structure is collinear, having a significantly reduced moment. Further Br substitution leads to a linear increase in TN up to x = 1. These results indicate that (CuCl)LaNb2O7 is located in the vicinity of the quantum phase boundary.