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Low-temperature structural investigations of the frustrated quantum antiferromagnets Cs2CuCl(4-x)Br(x)
Powder X-ray diffraction (PXRD) and single-crystal neutron scattering were
used to study in detail the structural properties of the Cs2CuCl(4-x)Br(x)
series, good realizations of layered triangular antiferromagnets. Detailed
temperature-dependent PXRD reveal a pronounced anisotropy of the thermal
expansion for the three different crystal directions of the orthorhombic
structure without any structural phase transition down to 20 K. Remarkably, the
anisotropy of the thermal expansion varies for different , leading to
distinct changes of the geometry of the local Cu environment as a function of
temperature and composition. The refinement of the atomic positions confirms
that for x=1 and 2, the Br atoms occupy distinct halogen sites in the
[CuX4]-tetrahedra (X = Cl, Br). The precise structure data are used to
calculate the magnetic exchange couplings using density functional methods for
x=0. We observe a pronounced temperature dependence of the calculated magnetic
exchange couplings, reflected in the strong sensitivity of the magnetic
exchange couplings on structural details. These calculations are in good
agreement with the experimentally established values for Cs2CuCl4 if one takes
the low-temperature structure data as a starting point