NMR and magnetization measurements in Li2VOSiO4 and Li2VOGeO4 are reported. The analysis of the susceptibility shows that both compounds are two-dimensional S = 1/2 Heisenberg antiferromagnets on a square lattice with a sizable frustration induced by the competition between the superexchange couplings J1 along the sides of the square and J2 along the diagonal. Li2VOSiO4 undergoes a low-temperature phase transition to a collinear order, as theoretically predicted for J2/J1>0.5. Just above the magnetic transition the degeneracy between the two collinear ground states is lifted by the onset of a structural distortion.
The isostructural compounds of general formula Cu2Te2O5X2 (X = Cl, Br) have been
synthesized by chemical transport reaction. They crystallize in the tetragonal system, space
group P4̄ with a = 7.6209(3) Å, c = 6.3200(4) Å, and a = 7.8355(2) Å, c = 6.3785(2) Å for
Cu2Te2O5Cl2 and Cu2Te2O5Br2 respectively. The three-dimensional network is built up by
tetrahedral clusters of copper(II) in distorted square planar coordination, isolated in the
[100] and [010] directions by [Te2O5E2] groups and in the [001] direction by [TeO3E]
tetrahedra. This leads to what can be described as spin tubes in the [001] direction
surrounded by large tunnels where 5s2 tellurium(IV) lone pairs and chlorine and bromine
atoms interact. A careful analysis of the temperature dependence of the susceptibility reveals
that even from the magnetic point of view, the building bricks are tetrahedra with all
couplings roughly equivalent, and not squares with small diagonal couplings. These systems
are thus expected to have very interesting magnetic properties, including a large singlet−triplet gap, and a band of singlet states inside this gap.
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