Low-temperature synthesis and structural analysis of novel tungsten oxide compounds

Low temperature hydrothermal synthesis of acidified tungstate solutions leads to the formation of defect pyrochlores MW{dollar}sb2{dollar}O{dollar}sb{lcub}6.50{rcub}cdot{dollar}yH{dollar}sb2{dollar}O, and compounds with the hexagonal tungsten bronze (HTB) structure, M{dollar}sb{lcub}rm x{rcub}{dollar}WO{dollar}sb{lcub}rm 3+x/2{rcub}{dollar}, where M is a monovalent cation. The initial pH of the tungstate solution was critical to the structure obtained; a pH of 3.5 leads to the formation of a pyrochlore phase and pH 1.5 gave the hexagonal tungsten bronze structure. The HTB compound was formed with lithium or sodium in the hexagonal tunnel, whereas the pyrochlore contains sodium, cesium, or rubidium inside the three dimensional tunnels. The phases are metastable and decompose around 500{dollar}spcirc{dollar}C. The sodium ions in the pyrochlore phase can be exchanged for many monovalent cations and in the hexagonal phase by hydrogen. The pyrochlore and HTB structures readily intercalate lithium.; Ammonium paratungstate has been found to reduce to an ammonium tungsten bronze when hydrothermally treated at 200{dollar}spcirc{dollar}C in non-aqueous solvents, such as acetic acid. The reduced phase has the hexagonal bronze structure with lattice parameters similar to the high-temperature phase.; Powder neutron diffraction and Rietveld analysis were used to investigate the crystal structure of hydrated and deuterated samples of Na{dollar}sb{lcub}rm x{rcub}{dollar}WO{dollar}sb{lcub}rm 3+x/2{rcub}cdot{dollar}yH{dollar}sb2{dollar}O (x {dollar}sim{dollar} 0.17, y {dollar}sim{dollar} 0.22). The compound, which crystallizes with the symmetry of space group P6/mmm, is related to the HTB structure, but differs from it due to the presence of sodium and oxygen along the hexagonal tunnel. Some of the oxygen appears in the structure as water molecules. The oxygen is disordered along the z axis in the hexagonal cavity with sodium in the hexagonal window.; The pyrochlore compound, Na{dollar}sb{lcub}1.0{rcub}{dollar}W{dollar}sb2{dollar}O{dollar}sb{lcub}6.50{rcub}cdot{dollar}1.0H{dollar}sb2{dollar}O, was investigated using powder neutron diffraction and Rietveld analysis. Structural refinement showed that the tungsten atoms on the 16c site and oxygen on the 48f site form the classical pyrochlore framework. A water oxygen was found on the 32e site with the deuterium atoms on a 96g site 1.10A away from the oxygen atom. A deuterium atom was found on the 192i site attached to a framework oxygen.