Raman Scattering and Fourier Transform Infrared Spectroscopy of Me6Al2(OH)16Cl2·4H2O (Me = Mg, Ni, Zn, Co, and Mn) and Ca2Al(OH)6Cl·2H2O Hydrotalcites

Hydrotalcites of the general formula Me 6 Al 2 (OH) 16 Cl 2 ·4H 2 O (Me = Mg, Ni, Zn, Co, and Mn) and Ca 2 Al(OH) 6 Cl·2H 2 O were prepared by the coprecipitation method. X-ray diffraction, thermal analysis, Fourier-transform infrared spectroscopy, and Raman scattering were employed to study the structural features of these materials. Particularly, the influences of different chemical compositions on the vibrational modes (stretching, bending, and lattice) were studied in detail. The compounds were produced with different crystallinity, ordering degree, and number of water molecules inside interlayers. Zinc hydrotalcite seems to be the most crystalline among the investigated materials without stacking faults, while Mg and Ni materials presented the lower ordering degree, crystallite sizes, and additional stacking faults. FTIR and Raman results demonstrated evidence of a complex band structure strongly correlated to the cationic radius, particularly, the lattice vibrations at lower frequencies. Stretching, bending, and lattice mode frequencies showed a decreasing tendency for increasing ionic radii, which are related to decreasing bond strengths. These results could be related to strongly coordinated water and chloride ions beside highly structured hydroxide layers, which present a direct influence on the chemical stability of the hydrotalcites.