Structural, infrared, and magnetic characterization of the solid solution series Sr2-xPbx(VO)(VO4)2;: Evidence of the Pb2+ 6s2 lone pair stereochemical effect

The solid solution Sr2-xPbxV3O9, 0 less than or equal to x less than or equal to 2, was prepared by solid state reactions and characterized by X-ray diffraction, IR spectroscopy, and magnetic susceptibility measurements. Single crystals of the pure strontium phase and mixed Sr/Pb compounds were prepared by high temperature treatment of the respective powder compositions. Pb2V3O9 crystals could only be obtained by the electrochemical reduction of molten PbV2O6. These crystals were always twinned. The previously reported crystal structure of Sr2V3O9 was confirmed. It was refined to R = 0.050, R-w = 0.057, in space group C2/c, a = 7.555(1) Angstrom, b = 16.275(2) Angstrom, c = 6.948(1) Angstrom, beta = 119.78(1)degrees The single crystal structural studies of the Sr1.02Pb0.98V3O9 and Sr0.67Pb1.33V3O9 members of the series show that the introduction of lead gives rise to a progressively complicated splitting of Sr2+/Pb2+ and the tetrahedral vanadium ion crystallographic sites. As a consequence the vanadium framework distorts and beyond the Sr0.5Pb1.5V3O9 composition the crystal symmetry becomes triclinic. This distortion is ascribed to the stereochemical effect of the 6s(2) lone pair of Pb2+. The crystallographic parameters of Pb2V3O9 are a = 7.598(1) Angstrom b = 16.393(3) Angstrom, c = 6.972(2) Angstrom, alpha = 91.38(1)degrees, beta = 119.35(1)degrees, gamma = 90.47(1)degrees. Pb2V3O9 exhibits a more complex IR spectrum than the monoclinic phases. Despite the similarity between the triclinic and monoclinic phases the magnetic susceptibilities indicate differences in the coupling between V4+ ions at low temperatures. (C) 1998 Academic Press.