Iron-promoted phases in the tungsten-oxygen system

An experimental survey of preparative work in the WO 3 Fe system in which temperatures up to 1100°C and pressures up to 55 kbar have been employed is presented. Two structures not usually found in the WO 3 or FeWO 3 system are encountered. Under the influence of high pressures the concentration of iron in an iron-tungsten bronze phase has been extended beyond the previously known limit of Fe 0.02 WO 3 to approximately Fe 0.1 WO 3 , which has a hexagonal structure with a = 7.422 ± 0.003A, c = 3.766 ± 0.002 A. In the absence of pressure but after long heating times at 1100°C and in the presence of small concentrations of iron, compositions close to W 5 O 14 yield a tetragonal phase a = 23.33 ± 0.01 A, c = 3.797 ± 0.001 A. The details of a single-crystal X-ray structural analysis of this phase are presented which show it to contain pentagonal and hexagonal tunnels and to be isostructural with Mo 5 O 14 as seen in |001| projection. The space group is identified as P 42 1 m which is different from that of Mo 5 O 14 . The difference between W 5 O 14 and Mo 5 O 14 is seen in the presence of less severe puckering and a simpler arrangement of the metal atoms about z = 1 2 which does not give rise to a superlattice cell hence leading to the observed difference in space group.