zigzag-chain structure and ferromagnetism of

The crystal structure of the insulator has been determined at 296 K by means of x-ray four-circle diffraction with residual factors of R = 0.027 and . The crystal data show an orthorhombic structure; space group Pnma, a = 14.301(1) Å, b = 3.598(1) Å, c = 5.204(1) Å, and Z = 4. The isolated V zigzag chains with a nearest-neighbour V-V distance of 3.05 Å are formed along the b-axis by sharing edges and corners of pyramids. The zigzag-chain unit and the pyramid are almost identical to those of the two-dimensional system which exhibits a simple dimer state with a gap of 660 K. The electronic states have been explored through magnetization and electron paramagnetic resonance measurements. is an ferromagnet with a Curie temperature . The paramagnetic properties above 50 K are explained in terms of the ferromagnetic Heisenberg chain model in the classical limit, where g = 1.96 and the edge-sharing exchange coupling , and the properties between 50 K and 30 K are considered as indicating a critical behaviour of the three-dimensional Heisenberg ferromagnet. This evidence is consistent with a previous model for the spin-gap formation of , in which it was postulated that the exchange coupling between the zigzag chains is rather larger than those within the zigzag chain.