The magnetic properties of RuF5

The structural unit of the pentafluorides MF₅ (M = Ru, Rh, Os, Ir, Pt) consists of M₄F₂₀ tetramers with the transition elements occupying the corners of the rhombus. In the case of RuF₅ the magnetic susceptibility vs. temperature curve shows a broad maximum at T ? 40 K. Neutron diffraction experiments below and above this temperature show that this maximum does not correspond to a magnetic three-dimensional ordering. Theoretical calculations based on isolated tetra-nuclear clusters of identical $\text{S =}\text{3}\text{2}$ spins have been applied. The intracluster exchange interactions are antiferromagnetic. The fitting of the susceptibility data leads to an exchange constant of $\text{J}\text{k}\text{= ?8.3 K}$.