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Short-range order of Br and three-dimensional magnetism in (CuBr)LaNb2O7

We present a comprehensive study of the crystal structure, magnetic structure, and microscopic magnetic model of (CuBr)LaNb2O7, the Br analog of the spin-gap quantum magnet (CuCl)LaNb2O7. Despite similar crystal structures and spin lattices, the magnetic behavior and even peculiarities of the atomic arrangement in the Cl and Br compounds are very different. The high-resolution x-ray and neutron data reveal a split position of Br atoms in (CuBr)LaNb2O7. This splitting originates from two possible configurations developed by [CuBr] zigzag ribbons. While the Br atoms are locally ordered in the 'ab' plane, their arrangement along the 'c' direction remains partially disordered. The predominant and energetically more favorable configuration features an additional doubling of the 'c' lattice parameter that was not observed in (CuCl)LaNb2O7. (CuBr)LaNb2O7 undergoes long-range antiferromagnetic ordering at TN=32 K, which is nearly 70% of the leading exchange coupling J4~48 K. The Br compound does not show any experimental signatures of low-dimensional magnetism, because the underlying spin lattice is three-dimensional. The coupling along the 'c' direction is comparable to the couplings in the 'ab' plane, even though the shortest Cu--Cu distance along 'c' (11.69 A) is three times larger than nearest-neighbor distances in the 'ab' plane (3.55 A). The stripe antiferromagnetic long-range order featuring columns of parallel spins in the 'ab' plane and antiparallel spins along 'c' is verified experimentally and confirmed by the microscopic analysis.