Synthesis of novel compounds with the pyrochlore and hexagonal tungsten bronze structures

Abstract

Low-temperature hydrothermal synthesis of acidified tungstate solutions leads to the formation of defect pyrochlores, M W 2 O 6 O 1/2 · x H 2 O, and compounds with the hexagonal tungsten bronze (HTB) structure, M x WO 3+x/2 , where M is normally a monovalent cation. The initial pH of the tungstate solution is critical to the structure obtained; a pH of 3.5 leads to the formation of a pyrochlore phase and pH 1.5 gives the hexagonal tungsten bronze structure. The HTB compound is formed with lithium or sodium in the hexagonal tunnel whereas the pyrochlore contains sodium, cesium, or rubidium inside the three-dimensional tunnels. Both phases are metastable and decompose around 500°C. The sodium ions in the pyrochlore phase can be exchanged for many monovalent cations and those in the hexagonal phase by hydrogen. Both structures readily intercalate lithium.