A novel integrated X-ray powder diffraction (XRPD) and molecular dynamics (MD) approach for modelling mixed-metal (Zn, Al) layered double hydroxides (LDHs)

A combination of experimental (XRPD) and computational (MD-simulation) techniques was used for a detailed study of the structural, dynamic and hydration properties of the ZnAl layered double hydroxides (LDHs) of formula [Zn0.65Al0.35(OH)(2)]Cl(0.35)(.)yH(2)O (I) and [Zn0.65Al0.35(OH)(2)](CO3)(0.175)center dot yH(2)O (II), with y = 0.35 and 0.69, respectively. Our approach was based on a direct comparison made, for the first time, between the observed XRPD diffraction pattern and the MD-simulated pattern of each model that was considered. The XRPD curve is affected (reflection angles and line shapes) by dynamic and structural factors, and the interlayer distribution of the water content. Accordingly, its reproduction through MD modelling is the most suitable means of monitoring these properties of the material. Molecular modelling was performed by MD simulations of models of I and II built up through appropriate modifications of their interlayer total water contents and related distribution. The validated models, namely those that provided the best MD-simulated pattern of I and II, were then used to determine structures and features which have not been previously determined by conventional diffraction techniques. The calculated XRPD curves were corrected to take into account the size broadening effect. The approach provided structural data that matched well those obtained by Rietveld analyses for the usual parameters. Furthermore, the study was original in its capability to detect and thus to discriminate between the different interatomic distances Zn-O, Al-O, (AlAl)-Al-..., (ZnZn)-Zn-..., (ZnAl)-Al-... in the slabs. The MD-simulated models of I and II provided the best reproduction of the XRPD curves for total water of hydration contents of y = 0.25 and y = 0.69, respectively. In the case of II, a domain with a homogeneous distribution of total water contents predominated. Conversely, in the case of I, a nonhomogeneous distribution featured by two (or multiple) shared crystalline domains having different interlayer water contents was evidenced. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).