Abstract
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 (NATI) and the other the EFC (NATII) in closer proximity to the aluminosilicate framework. We show that NATI materials have a lower onset pressure of PIA than the NATII materials containing Rb and Cs as EFC. The onset pressure of amorphization (PA) of NATII 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.
Highlights
not directly exchanged cations from natrolite mineral (Na-NAT) (Na16Al16Si24O8016H2O) is a natural zeolite with small elliptic pores having a conjugate diameter of less than 4 Å
As silicone oil only transmits hydrostatic pressure up to near 5 GPa11 we only used data up to this pressure to determine bulk moduli. These values indicate that the bulk moduli of the NATI members Li-NAT and Na-NAT and the NATII members K-NAT and Rb-NAT (P2) and Cs-NAT(P2) are within error the same whereas the P(1) phases of K-NAT, Rb-NAT, and Cs-NAT are significantly different. (Figures 1b and 3a and Supplementary Table S2)
Our experiments unequivocally show that materials with the same aluminosilicate framework but different H2O-EFC topologies have different onset pressures of Pressure-induced amorphization (PIA) and linear dependencies on cation size
Summary
Na-NAT (Na16Al16Si24O8016H2O) is a natural zeolite with small elliptic pores having a conjugate diameter of less than 4 Å. Under pressure in water-containing fluids pressure-induced hydration www.nature.com/scientificreports/. Occurs due to an expansion and pore-opening in this auxetic material[8]. This behavior was established in Li-, K-, Rb-NAT while Cs-NAT reveals a distinct pressure-induced phase transition without pressure-induced hydration[9]. Computational studies by Kremleva et al using density functional theory were able to optimize the structures of Li+, Na+, K+, Rb+, Cs+ containing natrolites at ambient conditions and point to the importance of the EFC-H2O interactions and the strain energy of the aluminosilicate framework under pressure[10]
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