Structural and magnetic aspects of the nanotube system Na2-xV3O7

We present experimental results of low-temperature x-ray synchrotron diffraction, neutron-scattering, and very low-temperature (mK range) bulk measurements on the nanotube system Na2 V3 O7. The crystal structure determined from our data is similar to the previously proposed model, but also deviates from it in significant details. The structure comprises of nanotubes along the c axis formed by stacking units of two V rings buckled in the ab plane. The space group is P 3 and the composition is nonstoichiometric, Na2-x V3 O7, x=0.17. The thermal evolution of the lattice parameters reveals anisotropic lattice compression on cooling. Neutron-scattering experiments monitor a very weak magnetic signal at energies from -20 to 9 meV. Magnetic susceptibility, specific-heat measurements, and decay of remanent magnetization in the 30-300 mK range reveal that the previously observed transition at ≈76 mK is spin-glass like with no long-range order. Presented experimental observations do not support models of isolated clusters but are compatible with a model of odd-legged S=1/2 spin tubes possibly segmented into fragments with different lengths. © 2008 The American Physical Society.