Thermal decomposition products of hydrotalcite-like compounds: low-temperature metaphases

Synthetic hydrotalcite-like samples with composition [M-1-Al-2+(x)x(3+)(OH)(2)](CO3)(x/2) . nH(2)O, where M = Mg, Ni, x = 0.25 and 0.33, n = 2x and 1.5x, are studied by XRD, DTA, TG and IR spectroscopy after heating in the temperature interval 120-260 degrees C and rehydration in air and water. Structural models of the two metaphases obtained are proposed. Metahydrotalcite-D (HT-D) is formed at 140-180 degrees C by the reversible dehydration of the interlayer. Metahydrotacite-B (HT-B) is formed at temperatures 240-260 degrees C as a result of the dehydroxilation of a part of OH groups of the brucite-like layer and inclusion of two oxygenes from the CO3-group in the same layer. The HT-B has a specific crystal structure. Rehydration does not restore the initial structural state but leads to the formation of a phase (HT-B-r) characterized with increased thickness of the interlayer, high content of water and a low temperature of dehydration. The properties of HT-B depend on the ratio M2+:Al of the initial sample. The substitution Mg <----> Ni does not influence the properties of the metaphases in the low temperature region. (C) 1999 Kluwer Academic Publishers.