Synthesis, structure and magnetic properties of V 4O 9—A missing link in binary vanadium oxides

Abstract

V 4O 9: 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. V 4O 9 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 VO 6 distorted octahedron, a VO 5 pyramid and a VO 4 tetrahedron. The structure of V 4O 9 is very similar to that of vanadyl pyrophosphate (VO) 2P 2O 7 which has PO 4 tetrahedra instead of VO 4 tetrahedra. This indicates that V 4O 9 is a salt of pyro-ion [V 2O 7] 4-; (VO) 2V 2O 7. The magnetic properties of V 4O 9 have been investigated by magnetic susceptibility, high-field magnetization and inelastic neutron scattering measurements. V 4O 9 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 73 K. The magnetic susceptibility behavior suggests that V 4O 9 is a spin-1/2 dimer system with significant interdimer interactions, as opposed to (VO) 2P 2O 7, which is an alternating spin-1/2 chain system. This difference is thought to be due to the fact that VO 4-mediated interactions are considerably weaker than PO 4-mediated interactions.