Characterization of Bi₅Nb₃O₁₅ by refinement of neutron diffraction pattern, acid treatment and reaction of the acid-treated product with n-alkylamines

The structure of Bi₅Nb₃O₁₅ was investigated by refinement of the powder neutron diffraction pattern as well as by structural change through acid treatment and subsequent treatments of an acid-treated product with n-alkylamines. Rietveld refinement suggests that Bi₅Nb₃O₁₅ adopts a mixed-layer Aurivillius-related phase structure, [Bi₂O₂]+[NbO₄]+[Bi₂O₂]+[BiNb₂O₇] [Pnc2 (space group No. 30)] with a = 2.1011 (4), b = 0.5473 (1) and c = 0.5463 (1) nm. After the acid treatment of Bi₅Nb₃O₁₅ with 3 mol/L HCl, a new reflection (at 2.25 nm after drying at room temperature or at 1.89 nm after drying at 120 °C) appeared in the X-ray diffraction (XRD) pattern in addition to the reflections due to Bi₅Nb₃O₁₅. Upon acid treatment, a part of the Bi ions were lost and essentially no Nb ions were dissolved during acid treatment to give a Bi/Nb molar ratio of 1.4. The TG curves of the acid-treated product showed mass loss (ca. 4 mass%) in the range of 300-600 °C. It was also demonstrated that the particle shapes did not change upon acid treatment. The reaction of the acid-treated product (after drying at room temperature) with n-alkylamines led to a shift of the newly appearing reflection to a lower angle, and the d-value of the low-angle reflection increased linearly in accordance with the increment of the number of carbon atoms in n-alkylamines. These results indicate that the [Bi₂O₂] sheet in Bi₅Nb₃O₁₅ was partially leached by acid treatment to form a layered compound H₄BiNb₃O₁₁·xH₂O, capable of accommodating n-alkylamines in the interlayer space, and its anhydrous form, H₄BiNb₃O₁₁, upon drying. Based on the variation in the interlayer distance upon intercalation of n-alkylamines into the acid-treated product, the structure of the acid-treated product can be suggested to comprise alternately stacked protonated [BiNb₂O₇] and [NbO₄] sheets, a result consistent with the Rietveld refinement of Bi₅Nb₃O₁₅.