JOURNAL OF SOLID STATE CHEMISTRY 96, 31-47 (1992) Synthesis of Novel Compounds with the Pyrochlore and Hexagonal Tungsten Bronze Structures KENNETH P. REIS, A. RAMANAN, AND M. STANLEY WHITTINGHAM Department of Chemistry and Materials Research Center, State University of New York at Binghamton, Binghamton, New York 13902-6000 Received July 29, 1991 Low-temperature hydrothermal synthesis of acidified tungstate solutions leads to the formation of defect pyrochlores, MW20601/z 9 xHzO, and compounds with the hexagonal tungsten bronze (HTB) structure, MxWO3+x/2, where M is normallya monovalentcation. 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~ The sodium ions in the pyrochlore phase can be exchangedfor many monovalent cations and those in the hexagonal phase by hydrogen. Both structures readily intercalate lithium. r 1992 Academic Press,Inc. Introduction The tungsten oxides and their intercalated products MxWO3+~/2 have been extensively studied over the years because of their elec- trochemical and electronic properties, which make them attractive as active elec- trodes in electrochromic displays (1). Their structures permit the ready insertion of cat- ions such as hydrogen, lithium, and sodium. However, the kinetics of these reactions have been insufficient to allow for their use in active displays such as those required in watches, calculators, and computer screens. Recently, new interest has re- volved around their use in large-scale static displays such as windows, mirrors, and sun- glasses. Important to the kinetics of the insertion reaction and its reversibility is the crystal- 31 line structure of the tungsten oxides. The WO6 octahedral building blocks join at the corners, which results in a variety of crystal- line structures. In an effort to synthesize new compounds with interesting structural and ionic properties, we have employed a variety of low-temperature techniques. These techniques include mild hydrother- real synthesis, ion exchange, and chemical insertion reactions at ambient temperatures, e.g. using n-butyl lithium to lithiate a com- pound. These low-temperature synthetic techniques allow for the formation of novel compounds that cannot be synthesized by traditional high-temperature solid-state re- actions. Recently, we reported preliminary results on the hydrothermal synthesis and charac- terization of two novel sodium tungstates with the pyrochlore and hexagonal tungsten 0022-4596/92 $3.00 Copyright 9 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.