Progress toward Zeolite-Based Self-Luminous Sensors for Radioactive Isotopes such as ²⁰¹Tl and ¹³⁷Cs: Structures and Luminescence of Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A

Smart materials that both sorb and announce the presence of radioactive isotopes such as ²⁰¹Tl and ¹³⁷Cs are needed for human safety in environmental catastrophies. This study reports progress toward zeolite-based self-luminous sensors for radioactive isotopes. Hf⁴⁺ was introduced into zeolite A by treating Tl-A and Cs,Na-A with HfCl₄(g) under anhydrous conditions. The crystal structures of the products, Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A, were determined by single-crystal crystallography using synchrotron X-radiation, with compositional confirmation by SEM-EDX analysis. Their luminescence properties upon X-ray, UV, and proton beam irradiation were studied. In Hf,Cl,Tl-A, some Hf⁴⁺ ions are 3-coordinate with three framework oxygen atoms of 6-rings; octahedral HfCl₆^2- ions are at the very centers of 7% of the large cavities within Tl₁₄HfCl₆¹²⁺ clusters. HfCl₆^2- ions are also seen in Hf,Cl,Cs,Na-A; they similarly occupy 7% of the large cavities and are members of a Cs₁₁HfCl₆⁹⁺ continuum. The X-ray-induced luminescence spectra of Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A, and that of Hf,Cl,Tl-A induced by UV, are all broad bands from 300 to 720 nm, peaking near 400 nm. By comparisons with the corresponding spectra of Zr,Cl,Tl-A, Zr,Cl,Cs,Na-A, Cs₂HfCl₆, and Cs₂ZrCl₆, several luminescence mechanisms are proposed. These zeolite-based materials can be expected to emit light (be self-luminous) as radioactive isotopes exchange into them. (Graph Presented).