Tetrahedral clusters of copper(II): Crystal structures and magnetic properties of Cu2Te2O5X2 (X = Cl, Br)

The isostructural compounds of general formula Cu2Te2O5X2 (X = Cl, Br) have been synthesized by chemical transport reaction. They crystallize in the t etragonal system, space group P (4) over bar with alpha = 7.6209(3) Angstro m, c = 6.3200(4) Angstrom, and a = 7.8355(2) Angstrom, c = 6.3785(2) A for Cu2Te2O5Cl2 and Cu2Te2O5Br2 respectively. The three-dimensional network is built up by tetrahedral clusters of copper(II) in distorted square planar c oordination, isolated in the [100] and [010] directions by [Te2O5E2] groups and in the [001] direction by [TeO3E] tetrahedra. This leads to what can b e described as spin tubes in the [001] direction surrounded by large tunnel s where 5s(2) tellurium(IV) lone pairs and chlorine and bromine atoms inter act. 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 sma ll diagonal couplings. These systems are thus expected to have very interes ting magnetic properties, including a large singlet-triplet gap, and a band of singlet states inside this gap.