4.5 Article

Abrupt structural transformation in hydrotalcite-like compounds Mg1-xAlx(OH)2(NO3)x•nH2O as a continuous function of nitrate anions

Journal

JOURNAL OF PHYSICAL CHEMISTRY B
Volume 105, Issue 9, Pages 1743-1749

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/jp0029257

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We report here a systematic XRD/FTIR/CHN/ICP investigation for structural transformation 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 finely divided steps in x value, a clear relationship between basal spacing and 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. transformation. Unlike that of normal hydrotalcites, the relationship revealed in our nitrated Mg1-xAlx(OH)(2)(NO3)(x). nH(2)O samples is much more complicated. While the lattice parameter a decreases with increase of nitrate content, the parameter c decreases slightly over x = 0.18-0.22, rises abruptly with a further increase of nitrate anions at x = 0.26, and then reaches maximums at x = 0.31-0.34. The compound at x = 0.26 also shows a sharp decrease in mean crystallite dimension. The abrupt structural change is attributed to a drastic change in nitrate anion arrangement. On the basis of our FTIR observations and Coulombic energy estimation, a stick-lying model for nitrate arrangement has been proposed, which is apparently superior to a tilt-lying model reported previously. The saturated charge density of the brucite-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 nitrate configuration from flat-lying to stick-lying due to increasing lateral repulsion among the anions and thus discontinuation in the planar growth.

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