SYNTHESIS AND CHARACTERIZATION OF COBALT(II)-IRON(III) HYDROXIDE CARBONATE, A LAYERED DOUBLE HYDROXIDE BELONGING TO THE PYROAURITE GROUP

A pure pyroaurite-type compound (PTC) with the composition [Co5.42IIFe 2.47III(OH)16][(CO3)1.12xH2O] (x = 5-6) has been synthesized by air ox idation of Fe(II) in a solution of Co(NO3)2 at pH 6.60 and has been ch aracterized by powder X-ray diffraction, electron microscopy, Fourier transform infrared spectroscopy, Mossbauer spectroscopy, and different ial thermal analysis. This reddish-brown compound consists of mostly h exagonally shaped crystals (diam. 0.1-0.4 mum) apparently more or less intergrown and forming spherical aggregates (diam. 3-5 mum). The hexa gonal unit-cell parameters are a = 0.312 and c = 2.278 nm, and there i s an ordered stacking of consecutive hydroxide layers. On heating in a nitrogen atmosphere, two DTA endothermic peaks are observed at 195 an d 260-degrees-C, respectively; the low-temperature peak is due to deso rption of interlayer water, whereas decomposition of carbonate and deh ydroxylation of the structure causes the high-temperature peak. Mossba uer parameters (delta = 0.35 mm s-1 and DELTAE(Q) = 0.49 mm s-1 at 298 K) are consistent with ferric ions in the high spin state. From the I R spectrum, the D3h symmetry of the interlayer CO3(2-) appears unpertu rbed. (C) 1994 Academic Press, Inc.