Synthesis, Diffusion and Ion-Exchange in Open Structure Sodium Tungstates and YBaCu Tungstates
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SYNTHESIS, DIFFUSION AND ION-EXCHANGE IN OPEN STRUCTURE SODIUM TUNGSTATES AND YBaCu TUNGSTATES
KENNETH P. REIS, A. RAMANAN, W. GLOFFKE and M. STANLEY WHITTINGHAM State University of New York at Binghamton, Materials Research Center and Chemistry Department, Binghamton, NY 13902-6000, USA. ABSTRACT Critical to the effectiveness of any solid state device is the reactivity of its components. In solid state ionics one desires certain atoms or ions to show high ionic mobility; yet, at the same time one does not want these atoms or ions to participate in side reactions. These reactions are a function of the crystalline structure of the material, of the way in which it was synthesized and of it's thermodynamic stability relative to the environment. This paper describes the synthesis of a variety of tungsten oxides which exhibit ionic mobility, and the determination of their crystalline structure and reactivity. The reactivity of the 123 superconductor with tungsten oxides is described in terms of the phases formed; none of these phases exhibited superconductivity. Both of these structurally related materials react with lithium and oxygen in a reversible manner, and these reactions are critically important in determining their properties and potential commercial application. INTRODUCTION The tungsten oxides have been extensively studied because of their possible use as the cathode of electrochromic devices [1]. Their structures permit the ready insertion of cations such as hydrogen, lithium and sodium. However, the kinetics of these reactions have been insufficient to allow their use in active displays such as those required in watches, calculators and computer screens. Recently new interest has revolved around their use in large scale static displays such as windows, mirrors and sunglasses. Important to the kinetics of the insertion reaction and its reversibility is the crystalline structure of the tungsten oxides. In an effort to synthesize new compounds with interesting structural and ionic properties, we have employed a variety of techniques. These techniques include mild hydrothermal synthesis, ion-exchange and chemical insertion reactions at ambient temperatures, e.g. using n-butyl lithium to lithiate a compound. These low temperature synthesis techniques have become increasingly popular in solid state chemistry as they allow for the formation of novel compounds that cannot be synthesized by traditional solid state reactions. Recently, we reported [2,3] the hydrothermal synthesis and characterization of two novel sodium tungstates with the pyrochlore and hexagonal tungsten bronze type (HTB) structure. The composition of these phases are Na0 .3 WO 3 .15 0.5H 2 0 and Na 1 .lW 2 06.55 1.4H 2 0 for the hexagonal and pyrochlore phases respectively. These compounds are fascinating because sodium is incorporated inside the tunnels of both structures and under high temperature synthesis sodium is too small to stabilize these structures. Further, both compounds contain large amounts of water and for the HTB structure more th
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