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The arrangement of encapsulated Ar atoms in the molecular-dimensioned cavities of fully dehydrated zeolite A of unit-cell composition Cs₃Na_8HSi₁₂Al₁₂O_(48) (Cs₃ -A) has been studied crystallographically to probe the confinement effect of guest species in microporous environment. Atoms of Ar were encapsulated in the cavities of Cs₃-A by treatment with 410 atm of Ar at 400 ℃ for two days, followed by cooling at room temperature. The crystal structure of Cs₃Na_8H-A(4Ar) (Pe = 410 atm, α = 12.245(2) A, R₁ = 0.0543, and R₂ = 0.0552) has been determined by single crystal X-ray diffraction technique in the cubic space group Pm 3 m at 21 (1) ℃ and 1 atm. Encapsulated Ar atoms are distributed in three crystallographic distinct positions: 1.5 Ar atoms per unit cell opposite 6-rings, 1.5 opposite four-rings in the large cavity, and finally 1.0 in the sodalite-unit. The possible structures of argon clusters, such as Ar₂, Ar₃, and Ar₄, are proposed.
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