  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.
