REMARKABLE STRUCTURE RELAXATION OF ZEOLITE WINDOWS IN RB-3-CENTER-DOT-A AND K-3-CENTER-DOT-A CRYSTAL-STRUCTURES OF M(3)NA(9-X)H(X)SI(12)AL(12)O(48), WHERE M=RB OR K AND X=1 OR 0

Four crystal structures of M(3)-A (M(3)Na(9-x)H(x)-A, M=Rb or K and x= 1 or 0), Rb3Na8H-A (a=12.228(1) Angstrom and R(1)=0.046), Rb3Nag-A (a = 12.258(3) Angstrom and R(1)=0.058), K3Na8H-A (a=12.257(3) Angstrom a nd R(1)=0.048) and K3Nag-A (a=12.257(3) Angstrom and R(1)=0.052), have been determined by single crystal x-ray diffraction technique in the cubic space group Pm3m at 21 degrees C. In all structures, each unit c ell contained three M(+) ions all located at one crystallographically distinct position on 8-rings. Rb+ ions are 3.12 and 3.21 Angstrom away respectively from O(1) and O(2) oxygens, about 0.40 <Angstrom away fr om the centers of the 8-rings, and K+ ions are 2.87 and 2.81 Angstrom apart from the corresponding oxygens. These distances are the shortest ones among those previously found for the corresoponding ones. Eight B-rings per unit cell are occupied by eight Nat ions, each with a dist ance of 2.31 Angstrom to three 0(3) oxygens. The twelfth cation per un it cell is found as Na+ opposite 4-ring in the large cavities of M(3)N a(9)-A and assumed to be H+ for M(3)Na(8)H-A. With these noble non-fra mework cationic arrangements, larger M(+) ions preferably on all large r 8-rings and the compact Na+ ions on all 6-ring, the bond angles in t he 8-rings of M(3 .)A, 145.1 and 161.0 respectively for (Si,Al)-O(1)-( Si,Al) and (Si,Al)-O(2)-(Si,Al), turned out to be remarkably stable an d smaller, by more than 12 to 170, than the corresponding angles found in the crystal structures of zeolites A with high concentration of M( +) ions. It is to achieve these remarkably relaxed 8-rings, the main w indows for the passage of gas molecules, with simultaneously maximized cavity volumes that MB-A have been selected as one of the efficient z eolite A systems for gas encapsulation.