Ceramics-Silikáty 49 (2) 126-131 (2005)

SYNTHESIS AND PROPERTIES OF FeNb₁₁O₂₉

P. Tabero

Using the XRD method a comparative study was performed on the synthesis reaction course for the FeNb₁₁O₂₉ phase taking as reactants the mixtures of a-Fe₂O₃ and H-Nb₂O₅, a-Fe₂O₃ and H-Nb₂O₅, FeNbO₄ and T-Nb₂O₅ or FeNb₄₉O₁₂₄ and a-Fe₂O₃. The above synthesis was also carried out by a solution method with the use of Fe(NO₃)₃ and ammonium niobium(V) oxalate solutions. It has been shown that independently of the kind of the used reacting substances an intermediate product of reaction or a product of a parallel reaction is FeNbO₄. The optimal method of synthesis with the use of solid reactants relies on the calcination of the a-Fe₂O₃/T-Nb₂O₅ mixture at 1000°C. The synthesis of FeNb₁₁O₂₉ by the solution method enables preparation of the pure product at 950°C. Independently of the starting substances, the monoclinic modification of FeNb₁₁O₂₉ is formed which at 1250°C undergoes a polymorphic transformation into the orthorhombic modification. The IR spectra recor-ded for both polymorphs of FeNb₁₁O₂₉ and H-Nb₂O₅, belonging to block-structure phases, in comparison to the spectra of a-Fe₂O₃ and FeNbO₄ contain additional absorption bands in the wavenumber range 1100-850 cm^-1. They are most probably due to stretching vibrations within octahedra containing very short M-O bonds, characteristic for the block-structure phases.

Keywords: FeNb₁₁O₂₉, XRD, IR, TG