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 WO(3) x 1.3 H(2)O (identical with 1/2H(2)W(2)O(7) x 1.6 H(2)O), using aqueous Na(2)WO(4) and SrCl(2). Our investigation shows that the concentration of HCl used to dissolve the SrCl(2) plays a crucial role in the stabilization of different structure types of layered TTHs. Highly acidic SrCl(2) (dissolved in 9 M HCl) solution yields an orthorhombic layered TTH of WO(3) x 2 H(2)O, while SrCl(2) dissolved in 3 M HCl appears to give an A-site-deficient 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(2)WO(4(aq)), 7.0 for SrCl(2(aq))). Previously, RP-type a DOLS of H(2)W(2)O(7) x 0.58 H(2)O was prepared, using an acid-leaching method, from the corresponding n = 2 member of the layered Aurivillius phase (AP) Bi(2)W(2)O(9). Powder X-ray diffraction showed the formation of layered RP DOLS with a large d spacing approximately 12.5 A, which is consistent with acid-leaching (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, as-precipitated 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 N(2).