One-Dimensional Spin Chains Attached to Molecular Orbitals in β -Phase Pb _x V Masashige Onoda

The crystal structure of β-phase Pb _x V ₂ O ₅ without superlattice reflection at x < 0.29 and the superlattice structure without diffuse scattering at 0.29 ≤ x ≤ 0.33 are precisely determined for x = 0.28 [analytical value = 0.2662(8)] and 0.33 [0.324(2)], respectively, indicating that the superlattice arises mainly from the partial ordering of Pb ions in the tunnels of the V ₂ O ₅ framework. The model for the two types of one-dimensional spin chains attached to the V–O–V molecular orbitals, where all V ions are in a mixed-valence state, is valid regardless of the presence or absence of Pb ion ordering. Long-time annealing of the crystal may induce short-range order between Pb ion layers only at x = 0.33, leading to the electron localization regarding transport and magnetic properties. The transport properties of x = 0.20 may be hopping conduction, those of 0.25 ≤ x ≤ 0.30 and the short-time annealed x = 0.33 have metallic phase, and those of long-time annealed x = 0.33 are nonmetallic. The composition dependence of thermoelectric power is explained with the spin chain model in the high-temperature limit. At x = 0.33, in the case of short-time annealing, only one of the two types of chains exhibits antiferromagnetic uniform chain magnetism with a nearly infinite chain length, whereas in the case of long-time annealing, both chains indicate the uniform chain magnetism.