Microscopic model for the frustrated Cu II -spin tetrahedron-based Cu4 Te5 O12 X4 (X=Cl, Br) systems

We present a microscopic study of the electronic and magnetic properties of the recently synthesized spin tetrahedron system Cu4 Te5 O12 Cl4 based on density functional calculations and on ab initio-derived effective models. In view of these results, we discuss the origin of the observed differences in behavior between this system and the structurally similar Cu2 Te2 O5 Cl2. Since the Br analog of the title compound has not been synthesized yet, we derive the crystal structure of Cu4 Te5 O12 Br4 by geometry optimization in an ab initio molecular dynamics calculation and investigate the effect of substituting Cl by Br. The possible magnetic behavior of Cu4 Te5 O12 Br4 in comparison with the recently studied Cu2 Te2 O5 Br2 is also discussed.