Exploration of Vanadate Selenites Solid Phase Space, Crystal Structures, and Polymorphism

摘要

Our recent exploration of the Pb-Ni-Se-O "crystallization phase diagram" under hydrothermal conditions has led to the discovery of three new selenites. Here we have applied a similar systematic method to the Pb-V-Se-O system using both hydrothermal and chemical vapor transport crystal growth using either PbO or PbCl2 as the source of lead. In our approach, phases in the presence are meticulously characterized by single crystal X-ray diffraction tests, on the basis of their morphology. Besides the determination of stable predominant phases in the diagram such as Pb-2(VO3)-, (SeO3)(2)Cl, it enables the crystallization of three novel vanadate selenites, beta-(V25+O3)(SeO3)(2) Pb-2(V4+O)(SeO3)(3) (II), and beta-Pb-4(V35+O8)(2)(SeO3)(3)(H2O) (III), depending on the method, pH, and potential of the solution. It is worth noting that in our conditions the solid/liquid crystal growth could be driven by kinetic or thermodynamic control which does not lead to determination of a formal phase diagram, but rather to a "solid phase space". The crystal structure of (I) is based on vanadate tetramers linked by selenite groups into a layer. The crystal structures of, and contain one-dimensional structural units composed of vanadate polyhedra and selnite groups interlinked through divalent read cations into a three-dimensional frameworks. The full panorama of reported lead vanadate selenites including our phases shows atypical polymorphic relations discussed on the basis of the structural complexity, very atypical among the polymorphic variations.