Antiferromagnetic Ordering of Magnetic Clusters Units in Nb₆F₁₅

We have studied the magnetic cluster compound Nb₆F₁₅ which has an odd number of 15 valence electrons per (Nb₆F₁₂)³⁺ cluster core, as a function of temperature using nuclear magnetic resonance, magnetic susceptibility, electron magnetic resonance and neutron powder diffraction. Nuclear magnetic resonance of the ¹⁹F nuclei shows two lines corresponding to the apical F^a-a nucleus, and to the inner Fⁱ nuclei. The temperature dependence of the signal from the Fⁱ nuclei reveals an antiferromagnetic ordering at T < 5 K, with a hyperfine field of ~2 mT. Magnetic susceptibility exhibits a Curie-Weiss behavior with T_N ~5 K, and μ_eff ~1. 57 μ_B close to the expected theoretical value for one unpaired electron (1. 73 μ_B). Electron magnetic resonance linewidth shows a transition at 5 K. Upon cooling from 10 to 1. 4 K, the neutron diffraction shows a decrease in the intensity of the low-angle diffuse scattering below Q ~0. 27 Å^-1. This decrease is consistent with emergence of magnetic order of large magnetic objects (clusters). This study shows that Nb₆F₁₅ is paramagnetic at RT and undergoes a transition to antiferromagnetic order at 5 K. This unique antiferromagnetic ordering results from the interaction between magnetic spins delocalized over each entire (Nb₆F₁₂ⁱ)³⁺ cluster core, rather than the common magnetic ordering.