Charge disproportionation and metal-insulator transition in the quasi-one-dimensional conductor β-Na_(0.33)V_2O_5: ~(23)Na NMR study of a single crystal

摘要

~(23)Na NMR measurements have been made on a single crystal to study the charge disproportionation accompanied by the metal-insulator transition in the quasi-one-dimensional conductor β-Na_(0.33)V_2O_5. We observed one set of ~(23)Na NMR spectrum split by an electric quadrupole interaction in the metallic phase with the Na disordering, two sets in the metallic phase with the Na ordering, and six sets in the insulating phase. These observations are concluded to be consistent with the lattice symmetry proposed by the x-ray diffraction and the neutron scattering studies. The angular dependences of the ~(23)Na Knight shift have been measured in the three phases. From the analysis of the angular dependences based on the local magnetic susceptibilities in the metallic phases determined by the ~(51)V NMR study, we can clearly conclude that the V1 site is the most magnetic. Furthermore, comparing those with the calculated curves based on several models of the charge ordering and the charge disproportionation in the insulating phase, we find that the metal-insulator transition is accompanied not by the charge ordering of V~(4+) and V~(5+) but by the charge disproportionation. We propose a model of the charge disproportionation consistent with the spin structure in the antiferromagnetic state. A model of the electronic structure in the β-Na_(0.33)V_2O_5 is also discussed from the aspect of the metal-insulator transition.