Crystal structures of encapsulates within zeolites. 3. Xenon in zeolite A

The positions of Xe atoms encapsulated in the cavities of fully dehydrated zeolite A of unit-cell composition Cs₃Na₈HSi¹²Al₁₂O₄₈ (Cs₃-A) have been determined. Cs3-A was exposed to 255 atm of xenon at 400 °C for 7 days, followed by cooling at pressure to encapsulate Xe atoms; a second crystal was treated similarly at 450 atm, and a third at 1020 atm. The resulting crystal structures of Cs₃-A(2.5Xe) (crystal 1, a = 12.245(2) Å, R₁ = 0.056, R₂ = 0.059), Cs₃-A(4.5Xe) (crystal 2, a= 12.258(2) Å, R₁ = 0.061, R₂ = 0.058), and Cs₃-A(5.25Xe) (crystal 3, a = 12.236(2) Å, R₁ = 0.061, R₂ = 0.057) were determined by single-crystal X-ray diffraction techniques in the cubic space group Pm3m at 21(1) °C and 1 atm. The approximately 2.5, 4.5, and 5.25 Xe atoms per unit cell, respectively, are distributed over three crystallographically distinct positions. At the center of each sodalite unit in crystals 1, 2, and 3, respectively, are 0.5, 0.75, and 0.75 Xe atoms at Xe(1); in addition, off center in crystal 3 are 0.25 Xe atoms at Xe(4), opposite a six-ring in the sodalite unit. Opposite four-rings in the large cavity at Xe(2) are 1, 2, and 2.25 Xe atoms, respectively, for the three crystals. Opposite six-rings in the large cavity at Xe(3) are 1, 1.75, and 2 Xe atoms, respectively. Relatively strong interactions of Xe(3) atoms with six-ring Na+ ions are observed: Na-Xe(3) = 3.18(2), 3.22(2), and 3.38(2) A, respectively. In Cs3-A(2.5Xe), two Xe atoms are found in each large cavity: Xe2 with a 4.54(4) A distance may exist. Cs₃-A(4.5Xe) may be viewed as a mixture of two kinds of unit cells: in most of their large cavities, four Xe atoms form a rhombus with Xe(2)-Xe(3) = 4.51(3) Å and Xe(2)-Xe(3)-Xe(2) = 95.1(5)°. Cs₃-A(5.25Xe) may also be viewed as having two kinds of large cavities: 75% contain a rhombus of four Xe atoms, Xe(2)-Xe(3) = 4.45(2) Å and Xe(2)-Xe(3)-Xe(2) = 97.3(4)°. The remaining 25% have five Xe atoms arranged trigonal-bipyramidally with three Xe(2)'s equatorial and two Xe(3)'s axial (Xe(2)-Xe(3) = 4.45(2) A). These arrangements of encapsulated Xe atoms in the large cavity are stabilized by alternating dipoles induced on Xe(2) and Xe(3) atoms by four-ring oxygens and six-ring Na⁺ ions, respectively.