Intralamellar structural modifications related to the proton exchanging in K4Nb6O17 layered phase

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Abstract

Basic structural aspects about the layered hexaniobate of K4Nb6O17 composition and its proton-exchanged form were investigated mainly by spectroscopic techniques. Raman spectra of hydrous K4Nb6O17 and H2K2Nb6O17·H2O show significant modifications in the 950–800 cm−1 region (Nb–O stretching mode of highly distorted NbO6 octahedra). The band at 900 cm−1 shifts to 940 cm−1 after the replacement of K+ ion by proton. Raman spectra of the original materials and the related deuterated samples are similar suggesting that no isotopic effect occurs. Major modifications were observed when H2K2Nb6O17 was dehydrated: the relative intensity of the band at 940 cm−1 decreases and new bands seems to be present at about 860–890 cm−1. The H+ ions should be shielded by the hydration sphere what preclude the interaction with the layers. Removing the water molecules, H+ ions can establish a strong interaction with oxygen atoms, decreasing the bond order of Nb–O linkage. X-ray absorption near edge structure studies performed at Nb K-edge indicate that the niobium coordination number and oxidation state remain identical after the replacement of potassium by proton. From the refinement of the fine structure, it appears that the Nb–Nb coordination shell is divided into two main contributions of about 0.33 and 0.39 nm, and interestingly the population, i.e., the number of backscattering atoms is inversed between the two hexaniobate materials.

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.

References (20)

  • M. Gasperin et al.

    J. Solid State Chem.

    (1980)
  • M. Gasperin et al.

    J. Solid State Chem.

    (1982)
  • M.A. Bizeto et al.

    Mater. Res. Bull.

    (2004)
  • U. Unal et al.

    J. Solid State Chem.

    (2006)
    S. Ida et al.

    Mol. Cryst. Liq. Cryst.

    (2007)
  • N. Miyamoto et al.

    J. Colloid Interface Sci.

    (2007)
  • K. Maeda et al.

    Chem. Mater.

    (2008)
  • M.A. Bizeto et al.

    J. Mater. Chem.

    (2009)
  • S.W. Keller et al.

    J. Am. Chem. Soc.

    (1994)
    M.A. Bizeto et al.

    Mater. Res. Bull.

    (2004)
    G.B. Saupe et al.

    Chem. Mater.

    (2000)
  • A.I. Ruiz et al.

    J. Nanosci. Nanotechnol.

    (2006)
  • M.A. Bizeto et al.

    Inorg. Chem.

    (2006)
There are more references available in the full text version of this article.

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