Structural and magnetic aspects of the nanotube system Na_2-xV₃O₇

Abstract

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 Na₂V₃O₇. The crystal structure determined from our data is similar to the previously proposed model [ P. Millet et al. J. Solid State Chem. 147 676 (1999)], 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 P3 ≈ and the composition is nonstoichiometric, Na_2-xV₃O₇, 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=½ spin tubes possibly segmented into fragments with different lengths.