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

摘要

High-resolution electron microscopy and diffraction studies of fully oxidized and reduced Pd-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 nB_8w_9o_47 ANALOGUE TO THE k_5nB_9w_2O_31 structure, with an orthorhombic approx √2a_TTB X √2a_TTB X c_TTB TTB superstructure having two PCs per cell. Electron diffraction determined the true structure as a 2a_TTB X 2a_TTB X 2c_TTB supercell. With further lead incorporation, leadfilled PTs replaced PCs, forming a quaternary TTB analogue. Reduced quaternary phases were prepared by reacting lead with Nb_8W_9O_47. The structure was initially maintained in the quaternary phase, but converted at high-temperature to the K_5Nb_9W_2O_47. The structure was initially maintained in the quaternary phase, but converted at high-temperature to the K_5Nb_9W_2O_31 type. Excess lead caused the formation of lead-rich interfaces between domains. Reaction of lead with oxides in the ratio 4Nb_2O_5:9WO_3 formed quaternary phases possessing the Nb_8W_9O_47 structure, demonstrating the direct synthesis of mixed PT/PC quaternary "bronze" phases which retained the structure of the oxidized ternary parent.