The crystal structures of natrolite and its dehydrated high-temperature phases (α1- and α2-metanatrolite) have been determined from powder X-ray diffraction measurements as a function of temperature and partial pressure of water (PH2O) to characterize the phase transition behavior. The evolution of crystal structure as a function of temperature shows two different phase transitions, depending on the PH2O, with α1-metanatrolite occurring at elevated PH2O and α2-metanatrolite occurring at low PH2O. Our discovery of α2-metanatrolite implies the existence of more than one transition mechanism, which we correlate with the migration of Na+ ions and the rate of evolution of H2O molecules. The transition behavior is rationalized in terms of two cooperating mechanisms: (1) dehydration-induced processes, which determine the phase transition temperature; and (2) thermally induced processes, which determine how the framework and its extraframework cations are modified.

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