Pressure-Induced Amorphization of Small Pore Zeolites-the Role of Cation-H₂O Topology and Anti-glass Formation

Systematic studies of pressure-induced amorphization of natrolites (PIA) containing monovalent extra-framework cations (EFC) Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ allow us to assess the role of two different EFC-H₂O configurations within the pores of a zeolite: one arrangement has H₂O molecules (NAT I) and the other the EFC (NAT II) in closer proximity to the aluminosilicate framework. We show that NAT I materials have a lower onset pressure of PIA than the NAT II materials containing Rb and Cs as EFC. The onset pressure of amorphization (P A) of NAT II materials increases linearly with the size of the EFC, whereas their initial bulk moduli (P1 phase) decrease linearly. Only Cs-and Rb-NAT reveal a phase separation into a dense form (P2 phase) under pressure. High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) imaging shows that after recovery from pressures near 25 and 20 GPa long-range ordered Rb-Rb and Cs-Cs correlations continue to be present over length scales up to 100 nm while short-range ordering of the aluminosilicate framework is significantly reduced-this opens a new way to form anti-glass structures.