High pressure synthesis of ammonium tungsten bronze superconducting phases from a molecular solid acid H3PW12O40•nH2O (n ∼28) in BN crucibles

A polyoxometalate molecular compound H3PW12O40 center dot nH(2)O (n similar to 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)(x)WO3 (x similar to 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 superconcluctivity below 5.2 K, which is the highest transition temperature in ammonium tungsten bronze phases reported in the literature. (C) 2008 Elsevier B.V. All rights reserved.