Antiferromagnetic Ordering of Magnetic Clusters Units in Nb(6)F(15).

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 Fa−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 μB close to the expected theoretical value for one unpaired electron (1.73 μB). 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 A−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 (Nb6F 12 i )3+ cluster core, rather than the common magnetic ordering.