Quaternary Lead–Niobium–Tungsten Oxides Based on the Tetragonal Tungsten Bronze Structure

ISSN： 0022-4596

Source： Journal of Solid State Chemistry, Vol.161, Iss.1, 2001-10, pp. : 135-151

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Abstract

High-resolution electron microscopy and diffraction studies of fully oxidized and reduced Pb–Nb–W–O oxides are presented. The phases observed were superstructures of tetragonal tungsten bronze (TTB), formed by ordering of pentagonal columns (PCs) and pentagonal tunnels (PTs). In fully oxidized preparations, the structure progressed from a quaternary Nb₈W₉O₄₇ analogue to the K₅Nb₉W₂O₃₁ structure, with an orthorhombic ∼√2a_TTB×√2a_TTB×c_TTB TTB superstructure having two PCs per cell. Electron diffraction determined the true structure as a 2a_TTB×2a_TTB×2c_TTB supercell. With further lead incorporation, lead-filled PTs replaced PCs, forming a quaternary TTB analogue. Reduced quaternary phases were prepared by reacting lead with Nb₈W₉O₄₇. The structure was initially maintained in the quaternary phase, but converted at high-temperature to the K₅Nb₉W₂O₃₁ type. Excess lead caused the formation of lead-rich interfaces between domains. Reaction of lead with oxides in the ratio 4Nb₂O₅:9WO₃ formed quaternary phases possessing the Nb₈W₉O₄₇ structure, demonstrating the direct synthesis of mixed PT/PC quaternary “bronze” phases which retained the structure of the oxidized ternary parent.