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Abstract: V4O9: A missing link of Wadsley phases has been successfully synthesized by using sulfur as a reducing agent at a low temperature and its structure has been determined by combining electron, X-ray and neutron diffractions. V4O9 has an orthorhombic Cmcm structure and the lattice parameters are a=10.356(2)Å, b=8.174(1)Å and c=16.559(3)Å at room temperature. The structure is composed of shared edges and corners of three types of polyhedra; a VO6 distorted octahedron, a VO5 pyramid and a VO4 tetrahedron. The structure of V4O9 is very similar to that of vanadyl pyrophosphate (VO)2P2O7 which has PO4 tetrahedra instead of VO4 tetrahedra. This indicates that V4O9 is a salt of pyro-ion [V2O7]4-; (VO)2V2O7. The magnetic properties of V4O9 have been investigated by magnetic susceptibility, high-field magnetization and inelastic neutron scattering measurements. V4O9 is a quantum spin system with a spin-gapped ground state. The excitation gap between the singlet ground state and the excited triplet state is approximately 73K. The magnetic susceptibility behavior suggests that V4O9 is a spin-1/2 dimer system with significant interdimer interactions, as opposed to (VO)2P2O7, which is an alternating spin-1/2 chain system. This difference is thought to be due to the fact that VO4-mediated interactions are considerably weaker than PO4-mediated interactions. [Copyright &y& Elsevier]