Crystal structure and magnetic properties of the novel oxide Pb2V5O12

The crystal structure of the new oxide Pb2V5O12 was determined from X-ray s ingle crystal and neutron powder diffraction data. The oxide is monoclinic, space group P2/c with a = 8.408(2), b = 5.017(1), c = 11.940(2) A. beta = 98.42(3)degrees, Z = 2. The crystal structure of Pb2V5O12 consists of isola ted layers formed by edge and corner sharing VO5 square pyramids with short vanadyl bonds directed above and below the layers. Six pyramids linked by corner form so-called "tiling plaquettes". Neighboring plaquettes are linke d by edge sharing. The Pb cations are located in the interstitial positions between layers. The structure of Pb2V5O12 is closely related to the (MVnO2 n+1)-O-IV homologous series known for Ca, Sr and Cd. Electron diffraction a nd high resolution electrom microscopy study confirm the crystal structure data and revealed a phase transition induced by irradiation resulting in th e formation of a disordered orthorhombic phase (a approximate to 3.75, b ap proximate to 3.79, c approximate to 5.03 Angstrom) which corresponds to a s ublattice of the plaquette-built structure. The analyses of related structu res allowed to suggest a more general formula for this homologous series: M mVm+nO3m+2n. The structures of such compounds may be described by shear pla nes formed by a shift of the VO2 units over a 1/2* [110] translation starti ng from the parent "VO2" or "MVO3" structures. Magnetic susceptibility meas urements of Pb2V5O12 indicate low dimensional behavior and a strong antifer romagnetic exchange within the plaquettes with J of the order of 190 K. Ano malies observed in the susceptibility and the specific heat indicate magnet ic order at T-N = 26 K.