Origin of the structural phase transitions and the anomalous electrical and magnetic properties of the magnetic metal NaV₆O₁₁

With decreasing temperature, the magnetic metal NaV₆O₁₁ undergoes two structural phase transitions (at 245 and 40 K) and exhibits anomalous electrical and magnetic properties. To probe the origin of these structural, electrical, and magnetic properties, the electronic structures of NaV₆O₁₁ were calculated for its crystal structures at room temperature, 200 K, and 30 K using the extended Hückel tight-binding band method. The 245 and 40 K structural phase transitions are not caused by a charge density wave instability, but by the lowering of the energy levels lying well below the Fermi level. In the magnetic metallic state of NaV₆O₁₁, obtained by the spin-polarization of the partially filled d-block bands, the electrical conductivity of NaV₅O₁₁ is predicted to be greater along the c-direction than in the ab plane, in agreement with experiment. Our study indicates that the unpaired electrons of NaV₆O₁₁ reside mainly in the V₃O₁₁ rather than in the V₃O₈ layers. The anomalies of the p_⊥-vs-T plot is explained by considering the effect of disordered magnetic moments on electrical conductivity.