Layered Tungsten Oxide-Based Hybrid Materials Incorporating Transition Metal Ions
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Layered Tungsten Oxide-Based Hybrid Materials Incorporating Transition Metal Ions Bridget Ingham1, S.V. Chong2 and Jeff L. Tallon1,2 1 Victoria University of Wellington, P.O. Box 600, Wellington, NEW ZEALAND 2 Industrial Research Limited, P.O. Box 31-310, Lower Hutt, NEW ZEALAND ABSTRACT Layered organic-inorganic hybrid materials based on tungsten oxide as the inorganic framework have been synthesised to include transition metal ions. The resulting materials have been characterised using a number of techniques. X-ray diffraction shows an interlayer expansion with increasing alkyl length. Infrared vibrational spectra of manganese tungstate compounds indicate the organic amine molecules are neutrally charged, and the inorganic framework is unaltered as one varies the organic intercalate. The magnetic behaviour of the materials has also been explored using a SQUID magnetometer. In the manganese tungstate hybrids an antiferromagnetic (AF) transition is observed, which decreases in temperature as the inorganic interlayer spacing is increased. A nickel tungstate hybrid sample, on the other hand, displays a ferromagnetic transition, which we attribute to a canted AF phase below 15 K. In all cases studied, the behaviour can be mapped to an effective moment (Peff) per transition metal ion, which agrees well with theoretical and literature values for other transition metal oxides.
INTRODUCTION Tungsten trioxide has been studied for a number of years due to its wide variation of electronic properties upon doping of the parent material. One can form tungsten bronzes by incorporating alkali metal ions such as sodium within the structure, which consists of cornershared octahedra in three dimensions, as in ReO3 or perovskite systems [1]. These bronzes have vivid colour and behave electrically as metallic conductors [2]. Due to the rapid colour change upon doping, tungsten trioxide has been used in applications such as electrochromic windows [3]. Tungsten trioxide can form in a variety of alternate phases, such as hexagonal, tetragonal or pyrochlore [1,4-5]. The hydrated form, commonly called “tungstic acid”, consists of layers of corner-shared WO6 octahedra separated by water molecules that are co-ordinated to alternate apices of the octahedra [6]. We have also found that it is also possible to intercalate organic molecules between the tungsten oxide layers, forming organic-inorganic hybrid materials [7]. In this paper we present recent results obtained from samples where the transition metal ions Mn, Fe, Co, Ni and Cu are added to the inorganic layer, creating metal tungstate hybrids.
EXPERIMENTAL Three methods were primarily used to form the transition metal tungstate hybrids. The first is an aqueous solution-based method similar to that used to produce tungsten oxide hybrids as reported earlier [7]: tungstic acid (H2WO4) is dissolved in aqueous ammonia solution (30 wt. %)
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and a solution of the appropriate organic amine, also dissolved in ammonia solution, is added to this with heating and stirring. Next
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