engleski

Molecular-to-Material Pathway: A Preparation of Ba-Nb Oxides from Metalorganic Framework

novel compound{; ; ; Ba2(H2O)5[NbO(C2O4)3] HC2O4}; ; ; .H2O (1) has been prepared and characterized by single-crystal XRD, IR and thermal analysis. The ability of 1 to act as a single-source precursor for the formation of bimetallic oxides has been explored by thermal analysis and XRPD. The thermal treatment of 1 at the chosen temperature (600-1200 °C) followed by cooling, results in the formation of the mixed-metal Ba5Nb4O15 and/or Ba4Nb2O9 oxide phases. The Ba5Nb4O15, as the major crystalline oxide phase, forms at ~700 °C. Three stable Ba4Nb2O9 polymorphs have been isolated: already well known hexagonal α-form [1-3] and orthorhombic γ-form [1-3], and so far unknown hexagonal δ-polymorph, having reduced symmetry 6H-perovskite structure. Heating 1 at 1135 °C and then cooling to RT leads to formation of α-Ba4Nb2O9, while the same procedure at 1175 °C results in the crystallization of another two polymorphs, γ-Ba4Nb2O9 and δ-Ba4Nb2O9. Electrical measurements were performed on samples prepared by pelleting milled single crystals of 1 and heated to chosen temperatures. The results of conductivity measurements were completely comparable with those reported for Ba4Nb2O9 ceramics prepared via multiple reheating steps, typical for conventional ceramics preparations [3]. Therefore, we believe that proposed synthetic procedure deserves additional attention since the benefit of “two in one” approach - one-step preparation of the desired oxide phase which is already in a form suitable for conductivity measurements - was successfully established. This study also focuses on controlling the phase composition and the crystallite domain lengths by altering preparation conditions, namely: (i) the time for which samples were held at the given temperature and (ii) the cooling rate. High temperature γ-Ba4Nb2O9 polymorph has been successfully retained and stabilized at room temperature (RT) ; desired crystallite size in nanoscale regime, ranging from ~5 to 20 nm, can easily be tuned. The crystallite domain length and lattice strain were calculated from X-ray diffraction (XRD) line broadening analysis performed during the Rietveld structure refinement. [1] Bezjak, J. ; Jančar, B. ; Rečnik, A. ; Suvorov, D. J. Eur. Ceram. Soc. (2008) 28 2771-2774. [2] Bezjak, J. ; Rečnik, A. ; Jančar, B. ; Boullay, P. ; Suvorov, D. J. Am. Ceram. Soc. (2009) 92 1806- 1810. [3] Ling, C. D. ; Avdeev, M. ; Kutteh, R. ; Kharton, V. V. ; Yaremchenko, A. A. ; Fialkova, S. ; Sharma, N. ; Macquart, R. B. ; Hoelzel M. ; Gutmann, M. Chem. Mater. (2009) 21 3853-3858.

Rietveld structural refinement; metal coordination complexes; metallic oxides

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