Structural Study of Sodium-Type Zeolite LTA by Combination of Rietveld and Maximum-Entropy Methods

Abstract

The electron-density distribution in hydrated and dehydrated sodium-type zeolite LTA was visualized from X-ray powder diffraction data by combining Rietveld analysis and a maximum-entropy method. The X-ray diffraction data were analyzed on the basis of the composition Na95Si97Al95O384 and a structural model (space group Fm3c) obtained by the Rietveld refinement of time-of-flight neutron powder diffraction data for the dehydrated sample. Weighted R factors, wRF, resulting from the analysis by the maximum-entropy method reached 1.15% for the hydrated sample and 0.99% for the dehydrated one. The number of water molecules per unit cell in the hydrated sample was estimated to be ca. 255 by electron-density analysis, agreeing well with ca. 248 determined by thermogravimetry. Adsorbed water molecules are situated beside Na+ ions in electron-density maps. In contrast with the dehydrated sample, Na+ ions in eight-membered rings were moved a little by introducing water into the α-cages.