Synthesis and Characterization of Ba2VO4 with the β-Ca2SiO4 Structure: Comparison with Sr2VO4

Phase-pure Ba2VO4 has been synthesized by hydrogen reduction of a mixture of Ba3V2O8 and 20% excess BaCO3 at 1200-1250°C, and its crystal structure, designated as β-Ba2VO4, refined using a combination of powder neutron and X-ray diffraction data. It is monoclinic, space group P 21/n (no. 14), with a = 6.0191(7) Å b = 7.6494(7), c = 10.465(1), β = 92.39(1)°, and V = 481.4(2) Å3 from Guinier X-ray diffraction data. β-Ba2VO4 is isostructural with β-Ca2SiO4. The structure is closely related to β-K2SO4 and consists of isolated VO4-4 tetrahedra and loosely bonded Ba(1)O10 and Ba(2)O9 polyhedra. Its magnetic behavior can be approximated as an S = 12 infinite Heisenberg linear chain. The crystal structure and magnetic properties of the β-Ba2VO4 are compared with those of the structurally related high-temperature form of Sr2VO4 (βSr2VO4). Reexamination of the crystal structure of the latter discloses that one of the V(IV) atoms has a rare trigonal bipyramidal coordination and that this atom is weakly bonded to a VO44 tetrahedron forming a V2O8-8 dimer. The formation mechanism of β-Ba2VO4 and the possible existence of an orthorhombic form isostructural with β-K2SO4 are also discussed.