Tetrahedral Clusters of Copper(II): Crystal Structures and Magnetic Properties of Cu₂Te₂O₅X₂ (X = Cl, Br)

The isostructural compounds of general formula Cu 2 Te 2 O 5 X 2 (X = Cl, Br) have been synthesized by chemical transport reaction. They crystallize in the tetragonal system, space group P 4̄ with a = 7.6209(3) Å, c = 6.3200(4) Å, and a = 7.8355(2) Å, c = 6.3785(2) Å for Cu 2 Te 2 O 5 Cl 2 and Cu 2 Te 2 O 5 Br 2 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 [Te 2 O 5 E 2 ] groups and in the [001] direction by [TeO 3 E] tetrahedra. This leads to what can be described as spin tubes in the [001] direction surrounded by large tunnels where 5s 2 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.