ON THE LACUNAR STRUCTURE OF PYROCHLORE-TYPE WO3

Pyrochlore-type tungsten trioxide has been prepared byH+/NH+4 exchange of ammonium pyrochlore-type tungstate. IR studies and density measurements of pyrochlore-type tungsten trioxide show the presence of H3O+ ions and W{double bond, long}O terminal groups in the oxide and an experimental density which is more than 20% lower than the calculated one. These findings have conducted us to investigate a structural lacunar model. A powder X-ray diffraction study led to a model with W and O vacancies and protons to maintain the global electroneutrality. This model has been refined by a powder neutron diffraction study, and protons have been localized in96h sites; the best model was obtained with an H0.65W1.635O5.23 formula with 18% and 13% of W and O vacancies, respectively, in comparison with stoichiometric WO3. H and O contents can vary under thermal treatment in the temperature range 100-350°C, due to reaction of protons with O of the framework. But stoichiometric pyrochlore WO3 can never be obtained because the reaction is complete only at about 400°C, the temperature at which the transformation pyrochlore WO3 → monoclinic WO3 occurs. © 1992 Academic Press, Inc.