Abstract

Synthesis of the Li-Al-Fe layered double hydroxides was performed by the coprecipitation method at constant pH (11.0 ± 0.2) and temperature (40 ± 2°C). Structural features of the as-synthesized samples were investigated by X-ray diffraction (XRD), infrared (IR) spectroscopy, scanning electron microscopy and Mössbauer spectroscopy. The samples consisted of well crystallized [LiFexAl2−x(OH)6]2CO3·nH2O phases with strict ordering of M+ and M3+ cations in the sheets. However, only a proportion of A13+ could be substituted by Fe3+ ions. The excess Fe3+ cations formed a separate ferrihydrite phase. Incorporation of Fe into the hydrotalcite-like structure resulted in an increase in the a lattice parameter determined by XRD. In addition, a shift of IR absorption bands, ascribed to the stretching vibrations of interlayer CO2−3 anions as well as the transitional motions of oxygen in the layers, to lower frequencies was observed. The presence of Fe3+ in the octahedral sheets caused a splitting of the band assigned to the stretching vibrations of the layer OH groups. Mössbauer experiments revealed that Fe exists in the synthesized samples in two different chemical environments. A proportion of the Fe3+ cations is incorporated as isolated ions in the [LiFexAl2−x(OH)6]2CO3·nH2O crystal structure. However, Fe3+ ions forming the ferrihydrite phase are dominant in the Fe-rich materials.

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