Intralamellar structural modifications related to the proton exchanging in K4Nb6O17 layered phase
Section snippets
1. Introduction
Layered niobates are an emerging class of solid state precursors for nanostructured materials preparation due to their semiconductor, structural and optical properties, which permit the assembling of new and interesting materials for different purposes [1]. The main studied layered niobate phase is the metal alkaline hexaniobate whose structure was detailed for Rb4Nb6O17[2], [3], K4Nb6O17[3] and Cs4Nb6O17[3]. All these niobate phases are isostructural and have negative charged slabs constituted
2.1. Preparation of the samples
K4Nb6O17 was prepared by ceramic method heating a stoichiometric mixture of Nb2O5 (Companhia Brasileira de Metalurgia e Mineração, CBMM, Brazil) and K2CO3 (Merck) at 1100 °C for 10 h, as previously described [9,10]. The heating process was made in two steps of 5 h each with one grinding between them. Crystal structure of hydrous K4Nb6O17 was confirmed by powder X-ray diffractometry (d040=0.94 nm). The acidic form was prepared by ion-exchange, refluxing a suspension of K4Nb6O17 in a 6 mol/L HNO3
3. Results and discussion
The vibrational spectra of hexaniobate are dominated by bands attributed to internal vibrational modes of distorted NbO6 octahedra (linked by corner and by edge sharing) in the 200–1000 cm−1 region and also by external vibrations of the crystal below 170 cm−1[12], [17]. Fig. 2 shows that Raman spectra between both hexaniobates I and II exhibit significant modifications in the following regions: 950–800 cm−1 (Nb–O terminal stretching mode of highly distorted NbO6 octahedra), 700–500 cm−1 (Nb–O
4. Conclusion
Slight structural modifications invisible by XRD are here evidenced by Raman spectroscopy. This is reinforced by direct observations obtained from XAS spectroscopy, and the refinement of the first cation shell (Nb–Nb backscattering) pictures an atomic arrangement closely related between both hexaniobate phases but accompanied with a slight distortion of the NbO6 chain having for effect to inverse the corner to edge-Oh neighbors.
Acknowledgement
The authors would like to thank the CAPES/COFECUB through the project 557/07 and also the Brazilian agencies FAPESP and CNPq for financial support and fellowships. The authors also acknowledge LNLS (project 4337/04) for the XAS facilities.
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