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

Using the XRD method a comparative study was performed on the synthesis reaction course for the FeNb₁₁O₂₉ phase taking as reactants the mixtures of α-Fe₂O₃ and T-Nb₂O₅, α-Fe₂O₃ and H-Nb₂O₅, FeNbO₄ and T-Nb₂O₅ or FeNb₄₉O₁₂₄ and α-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 α-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 recorded for both polymorphs of FeNb₁₁O₂₉ and H-Nb₂O₅, belonging to block-structure phases, in comparison to the spectra of α-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.