NICKEL-IRON HYDROXIDE CARBONATE PRECURSORS IN THE SYNTHESIS OF HIGH-DISPERSITY OXIDES

Nickel-iron hydroxide carbonates with layered reevesite-like structures have been synthesized by coprecipitation and their evolution to high-dispersity oxides has been studied. Ni-II-Fe-III cation substitution in the octahedral positions of the cationic layers for a wide interval of nickel:iron ratios (0.33-3) is deduced from the XRD and DTA data and the Mossbauer spectra. No divalent iron is present in the dried samples and no admixtures are detected in the diffraction patterns and the IR spectra. Thermal decomposition of the precursor hydroxide carbonates occurs at temperatures < 580 K and high-dispersity oxides are obtained. The average particle size of nickel ferrite, synthesized at 623 K is 7.5 nm and the specific surface area is 160 m(2) g(-1). A mixture of oxides is obtained from the precursors with iron:nickel ratios different from that of ferrite, in agreement with the phase diagram of that system. At 873 K a partly inverse nickel ferrite with the cation distribution Ni-0.1(5)Fe-0.8(5)[Fe-1.1(5)Ni-0.8(5)]O-4 is obtained. The surface composition of the oxide samples displays deviation from the bulk ratio in favour of nickel that increases with the iron:nickel ratio. For samples with Fe:Ni equal to or greater than 0.5 the oxygen-to-metal surface ratio is higher than that expected on the basis of the formula of spinels and sesquioxides.