THE COMPOSITION DEPENDENCE OF THE STRUCTURE OF POTASSIUM-LOADED ZEOLITE-A - EVOLUTION OF A POTASSIUM SUPERLATTICE

Three samples of potassium zeolite A, containing 1, 3, and 5 additional potassium atoms per primitive unit cell, were prepared through the controlled reaction of dehydrated potassium zeolite A with a calibrated amount of potassium vapor. The structures of these materials, and that of the host dehydrated K12-A, were determined by Rietveld refinement of powder neutron diffraction data. The structures of K12-A and the most lightly loaded sample with one extra potassium atom per unit cell (K1/K12-A) were refined in space group Pm3̄m assuming disorder of Si and Al. For the more heavily loaded samples K3/K12-A and K3/K12-A a superlattice was observed, resulting in a doubling of the unit cell in space group Fm3̄m . This superlattice arises as a result of potassium ordering. On potassium inclusion additional potassium sites K3 in the sodalite cage and K4 in the α cage are observed. At low loadings occupation of the site in the sodalite cages begins. The separation between the first of these two sites and K1 on the opposite of the 6-ring windows is such that the two sites cannot be occupied simultaneously, leading to occupation of the second new site facing the 4-rings of the α cage. The occupancies of these two new sites increase with increasing loading levels until ordering occurs. At limiting loading a tetrahedron of K3 forms in each sodalite cage, with alternate α cages containing 8 K1 and 12 K4, respectively. The possible role of K3+4 in directing the formation of the superlattice is discussed. © 1994 Academic Press. All rights reserved