Abrupt structural transformation in hydrotalcite-like compounds Mg1-xAlx(OH)(2)(NO3)(x)center dot nH(2)O as a continuous function of nitrate anions

We report here a systematic XRD/FTIR/CHN/ICP investigation for structural t ransformation of hydrotalcite-like compounds Mg1-xAlx(OH)(2)(NO3)(x). nH(2) O as a continuous function of nitrate anions (x = 0.18-0.34). With seven fi nely divided steps in x value, a clear relationship between basal spacing a nd nitrate anion content (and thus trivalent cation content) has been found . In particular, we have been able to "arrest" an intermediate hydrotalcite -like phase that situates at the midpoint of an abrupt structural. transfor mation. Unlike that of normal hydrotalcites, the relationship revealed in o ur nitrated Mg1-xAlx(OH)(2)(NO3)(x). nH(2)O samples is much more complicate d. While the lattice parameter a decreases with increase of nitrate content , the parameter c decreases slightly over x = 0.18-0.22, rises abruptly wit h a further increase of nitrate anions at x = 0.26, and then reaches maximu ms at x = 0.31-0.34. The compound at x = 0.26 also shows a sharp decrease i n mean crystallite dimension. The abrupt structural change is attributed to a drastic change in nitrate anion arrangement. On the basis of our FTIR ob servations and Coulombic energy estimation, a "stick-lying" model for nitra te arrangement has been proposed, which is apparently superior to a "tilt-l ying" model reported previously. The saturated charge density of the brucit e-like layers for the "stick-lying" nitrate anions is about 4 e/nm(2). The sharp decrease in crystallite dimension can be ascribed to the change of ni trate configuration from "flat-lying" to "stick-lying" due to increasing la teral repulsion among the anions and thus discontinuation in the planar gro wth.