Accidental extinction in powder XRD intensity of porous crystals: Mesoporous carbon crystal CMK-5 and layered zeolite-nanosheets

CMK-5 mesoporous carbon crystals with p6mm symmetry and different carbon-wall thicknesses were prepared using furfuryl alcohol. We clearly observed that the XRD intensity of the 10 reflection diminishes at a certain thickness of carbon-wall, i.e., an accidental extinction happens in CMK-5. In order to make clear the condition of the accidental extinction, XRD intensity is analytically formulated/calculated for two-dimensional system with p6mm symmetry. The conditions for the accidental extinction for the I I and 20 reflections as well as for the 10 reflection in CMK-5 were clarified. Pore diameter and carbon wall thickness were measured independently by N-2 gas adsorption isotherm and TEM images as a function of the carbon-loading, and we confirmed internal consistency between the observations and calculation for the accidental extinction. Similarly it was pointed out through analytical formulation for one-dimensional system such as layered zeolite-nanosheets that the accidental extinction could be observed for the order (I > I) at the certain conditions. It was shown as a whole that knowledge of occurrence of the accidental extinction not only will prevent from determining false symmetry but also will provide internal structural information. (C) 2009 Elsevier Inc. All rights reserved.