Neutron diffraction study on the antiferromagnetic insulating ground state of β-Na _0:33V ₂O ₅

The magnetic structure of the antiferromagnetically ordered state of a quasi-one-dimensional conductor β-Na _0:33V ₂O ₅ below T _N = 24K was examined by neutron diffraction. The presence of magnetic Bragg reflections at (h; k ± 1/6, 0), h + k = odd below T _N indicates that the amplitudes of the magnetic moments at the V sites in three types of V-O chains are modulated with a period of 3b along the chain direction (∥ b). On the other hand, it was confirmed that the lattice modulation along the b direction occurs at ∼244K and at ∼131 K with a period of 2b and 6b, respectively. Although the latter structural transition had been attributed to a "charge ordering" transition, which would result in a charge disproportionation in the V 3d orbitals with a period of 6b, the 3b periodicity in the amplitude of magnetic moments strongly suggests that the charge disproportionation occurs with a period of 3b, not 6b. The 6b lattice modulation developing below 131 K is therefore attributed to an instability in the V 3d band to form 3b charge modulation on the preexistent 2b lattice modulation.