Crystal structure and magnetic properties of the S = 1/2 quantum spin system Cu7(TeO3)6F2 with mixed dimensionality.

The new oxofluoride Cu7(TeO3)6F2 has been synthesized by hydrothermal synthesis. It crystallizes in the triclinic system, space group P1. The crystal structure constitutes a Cu–O framework with channels extending along [001] where the F– ions and the stereochemically active lone-pairs on Te4+ are located. From magnetic susceptibility, specific heat, and Raman scattering measurements we find evidence that the magnetic degrees of freedom of the Cu–O–Cu segments in Cu7(TeO3)6F2 lead to a mixed dimensionality with single Cu S = 1/2 moments weakly coupled to spin-chain fragments. Due to the weaker coupling of the single moments, strong fluctuations exist at elevated temperatures, and long-range magnetic ordering evolves at comparably low temperatures (TN = 15 K).