Revisiting tungsten trioxide hydrates (TTHs) synthesis - Is there anything new?

We report a very simple precipitation route to prepare a layered perovskite-type structure, tungsten trioxide hydrate (TTH), with the nominal chemical formula of WO₃-1.3H₂O (= 1/2H₂W ₂O₇-1.6H₂O), using aqueous Na ₂WO₄ and SrCl₂. Our Investigation shows that the concentration of HCl used to dissolve the SrCl₂ plays a crucial role In the stabilization of different structure types of layered TTHs. Highly acidic SrCl₂ (dissolved In 9 M HCl) solution yields an orthorhombic layered TTH of WO₃-2H₂O, while SrCl₂ dissolved in 3 M HCl appears to give an A-slte-deficlent Ruddlesdon-Popper (RP) related double-perovskite-type layered structure (DOLS-TTH). A well-known scheelite-type structure is obtained under weakly basic conditions (pH = 10.3 for Na ₂W0_4(aq), 7.0 for SrCl_2(aq)). Previously, RP-type a DOLS of H₂W₂O₇ • 0.58H ₂O was prepared, using an acid-leaching method, from the corresponding n = 2 member of the layered Aurlvilllus pjiase (AP) Bi ₂W₂O₉. Powder X-ray diffraction showed the formation of layered RP DOLS with a large d spacing -12.5 Á, which is consistent with acld-leachlng (Kuto et al. Inorg. Chem. 2003, 42, 4479-4484; Wang et al. J. Solid State Chem. 2007,180, 1125-1129) and exfoliation (Schaak et al. Chem. Commun. 2002, 706-707) methods for synthesized TTHs. The proposed DOLS-TTH structure of newly prepared TTHs was further confirmed by an intercalation reaction using n-octylamine (C8A). A transmission electron microscopy study showed the formation of nanosized particles, and scanning electron microscopy coupled with energy dispersive X-ray analysis showed the absence of Na and Sr In the air-dried, asprecipitated products under acidic conditions. The bulk electrical (proton) conductivity of presently prepared TTHs was found to be on the order of 10^-4-10^-3 S/cm at room temperature In wet N².