A layered tungstic acid H₂W₂O₇·nH₂O with a double-octahedral sheet structure: Conversion process from an aurivillius phase Bi₂W₂O₉ and structural characterization

The conversion process of an Aurivillius phase, Bi₂W₂O₉, into a layered tungstic acid by hydrochloric acid treatment has been investigated, and resultant H₂W₂O₇·7H₂O has been fully characterized. The c parameter of Bi₂W₂O₉ [2.37063(5) nm] decreases to 2.21(1) nm in an acid-treated product dried at ambient temperature. The a and b parameters of Bi₂W₂O₉ [a = 0.54377(1) nm and b = 0.54166(1) nm] also decrease slightly to a = 0.524(1) nm and b = 0.513(1) nm in the acid-treated product dried at ambient temperature, indicating structural changes in the ReO3-like slabs in Bi₂W₂O₉ upon acid treatment. Drying at 120 °C leads to a further decrease in the c parameter [1.86(1) nm] with no notable change in the a and b parameters [a = 0.5249(2) nm and b = 0.513(2) nm]. The formation of an expandable layered structure is demonstrated by the successful intercalation of n-octylamine [interlayer distance 2.597(9) nm] and n-dodecylamine [interlayer distance 3.56(2) nm]. The compositions of the acid-treated products are determined to be H₂W₂O₇·H₂O typically with n = 0.58 for the air-dried product and n = 0 for the product dried at 120 °C. As a consequence, the composition of the layer is H₂W₂O₇, and the decrease in the c parameter upon drying is ascribable to the loss of interlayer water. Scanning electron microscopy reveals no morphological change during acid treatment, which strongly suggests a selective teaching of the bismuth oxide sheets as a reaction mechanism. High-resolution transmission electron microscopy (HREM) observation of the acid-treated product shows consistency with a structural model for H₂W₂O₇, derived from Bi₂W₂O₉ through removal of the bismuth oxide sheets and contraction along the c axis. HREM observation also reveals that the WO₆ octahedra arrangement changes slightly with acid treatment. A one-dimensional electron density map projected on the c axis for the product dried at 120 °C, H₂W₂O₇, shows good consistency with that calculated for the structural model.