Synthesis and crystal structure of zinc iodide in the sodalite cavities of zeolite A (LTA)

The crystal structure of ZnI2 molecule synthesized in zeolite A (LTA) has been studied by single-crystal X-ray diffraction techniques. A single crystal of |Zn₆|[Si₁₂Al₁₂O₄₈]-LTA, synthesized by the dynamic ion-exchange of |Na₁₂|[Si₁₂Al ₁₂O₄₈]-LTA with aqueous 0.05 M Zn(NO₃) ₂ and washed with deionized water, was placed in a stream of flowing 0.05 M KI in CH₃OH at 294 K for four days. The resulting crystal structure of the product (|K₆Zn₃(KI)₃(ZnI ₂)_0.5|[Si₁₂Al₁₂O₄₈]-LTA, a = 12.1690(10) Å) was determined at 294 K by single-crystal X-ray diffraction in the space group Pm m. It was refined with all measured reflections to the final error index R₁ = 0.078 for 431 reflections which F_o > 4σ(F_o). At four crystallographically distinct positions, 3.5 Zn²⁺ and nine K⁺ ions per unit cell are found: three Zn²⁺ and five K⁺ ions lie on the 3-fold axes opposite 6-rings in the large cavity, two K⁺ ions are off the plane of the 8-rings, two K⁺ ions are recessed deeply off the plane of the 8-rings, and the remaining a half Zn²⁺ ion lie on the 3-fold axes opposite 6-rings in the sodalite cavity. A half Zn²⁺ ion and an I^? ion per unit cell are found in the sodalite units, indicating the formation of a ZnI₂ molecule in 50% of the sodalite cavities. Each ZnI₂ (Zn-I = 3.35(5) Å) is held in place by the coordination of its one Zn²⁺ ion to the zeolite framework oxygens and by the coordination of its two I^? ions to K⁺ ions through 6-rings (I-K = 3.33(8) Å). Three additional I^? ions per unit cell are found opposite a 4-ring in the large cavity and form a K ₃I²⁺ and two K₂ZnI³⁺ ionic clusters, respectively.