The solid solution Sr 2؊x Pb x V 3 O 9 , 04x42, 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. Pb 2 V 3 O 9 crystals could only be obtained by the electrochemical reduction of molten PbV 2 O 6 . These crystals were always twinned. The previously reported crystal structure of Sr 2 V 3 O 9 was confirmed. It was refined to R ؍ 0.050, R w ؍ 0.057, in space group C2/c, a ؍ 7.555(1) A > , b ؍ 16.275(2) A > , c ؍ 6.948(1) A > , ؍ 119.78(1)°. The single crystal structural studies of the Sr 1.02 Pb 0.98 V 3 O 9 and Sr 0.67 Pb 1.33 V 3 O 9 members of the series show that the introduction of lead gives rise to a progressively complicated splitting of Sr 2؉ /Pb 2؉ and the tetrahedral vanadium ion crystallographic sites. As a consequence the vanadium framework distorts and beyond the Sr 0.5 Pb 1.5 V 3 O 9 composition the crystal symmetry becomes triclinic. This distortion is ascribed to the stereochemical effect of the 6s 2 lone pair of Pb 2؉ . The crystallographic parameters of Pb 2 V 3 O 9 are a ؍ 7.598(1) A > , b ؍ 16.393(3) A > , c ؍ 6.972(2) A > , ؍ 91.38(1)°, ؍ 119.35(1)°, ؍ 90.47(1)°. Pb 2 V 3 O 9 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 V 4؉ ions at low temperatures.
1998 Academic Press
EXPERIMENTAL
Synthesis
Crystals of Pb
V O were grown by electrochemical reduction of PbV O fused at 650°C in an alumina crucible contained in a steel vessel. The electrolysis was performed for 6 h under flowing nitrogen. Nickel wire was used as the electrodes. The potential applied to the electrodes was set to 3 V, yielding an initial current of 200 mA that progressively increased to 500 mA after 6 h. The anode was attacked during the experiment, leading to a small diffusion of Ni in the melt which could explain the current change. Black parallepiped crystals were mechanically isolated from the 417