Antiferromagnetic Ordering of Magnetic Clusters Units in Nb6F15

We have studied the magnetic cluster compound Nb(6)F(15) which has an odd number of 15 valence electrons per (Nb(6)F(12))(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 (19)F nuclei shows two lines corresponding to the apical F(a-a) nucleus, and to the inner F(i) nuclei. The temperature dependence of the signal from the F(i) 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 Nb(6)F(15) 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 (Nb(6)F(12) (i))(3+) cluster core, rather than the common magnetic ordering.