Crystal structure of a sodium sorption complex of zeolite X containing linear Na-3(2+) clusters

An anhydrous single crystal of sodium zeolite X, Na92Si100Al92O384, was exposed for 48 h to Na vapor at 225 degrees C. Its structure was determined by X-ray diffraction methods in the cubic space group Fd (3) over bar; a = 25.100(11) Angstrom at 22 degrees C; R-1 = 0.071 and R-2 = 0.088 for the 544 reflections for which I > 3 sigma(I). A total of 106 sodiums are found per unit cell, 14 more than the number of Na+ ions required to balance the negative charge of the zeolite framework. The same number, 14, of Na atoms are found at the very centers of 12-oxygen rings, far from the zeolite framework. It appears that each Na atom has been captured by a pair of Na+ ions to form linear Na-3(2+) clusters that nearly fill the 12-oxygen rings. Each terminal sodium approaches two oxygens of an AlO4 tetrahedron of the zeolite framework with Na-O distances of 2.33(3) and 2.83(4) Angstrom. Although some delocalization of the single valence electron over this cluster must occur, it is recognized, from the sodium approach distances to framework oxygens, that the terminal sodiums are largely cationic and the central sodium is closer to neutral. The bond lengths in the cluster are 2.93(3) Angstrom, about the same as the corresponding distance in Na-5(4+), 2.80(2) Angstrom, and the sum of the radii of Na-0 and Na+, 2.83 Angstrom.