Abstract

A polyoxometalate molecular compound H₃PW₁₂O₄₀·nH₂O (n 28) was treated under high pressure and high temperature conditions (10 GPa, 873–1173 K) in BN crucibles. Powder X-ray diffraction, electron microprobe analysis, and infrared absorption spectroscopy confirmed the formation of (NH₄)_xWO₃ (x 0.26–0.34) as the main product. This compound was not formed when the source material was heated at ambient pressures in a BN cell, suggesting that the high pressure confined water molecules in the system and brought about the hydrolysis reaction on the BN crucible walls in the presence of the acid to generate ammonium cations in the cell. The (NH₄)_0.25WO₃ product formed at 973 K showed superconductivity below 5.2 K, which is the highest transition temperature in ammonium tungsten bronze phases reported in the literature.

Keywords: Heteropolyacid; Intercaration; Hydrothermal reaction

Article Outline

1. Introduction

2. Experimental

2.1. Materials and synthesis

2.2. Characterization of the samples

3. Results

3.1. Structure of the materials formed under high pressure

3.2. EPMA analysis and IR spectrum of the products

3.3. Magnetic measurements of the tungsten bronze phases

4. Discussion

4.1. Formation of (NH₄)_xWO₃ from H₃PW₁₂O₄₀·nH₂O heteropolyacid (HPA) under high pressure

4.2. Superconductivity of the tungsten bronze phases

5. Conclusions

Acknowledgements

References