Probing the dynamics of instability in zeolitic materials

Abstract

Zeolites collapse under modest pressure or temperature, their microporous structurestransforming into glasses of conventional density. Using in situ synchrotron radiationdiffraction methods we show how pressure and temperature-induced amorphization areequivalent processes and that these are mirrored by changes in the local structure of chargecompensating cations. Evidence for a low density amorphous phase and a high densityamorphous phase present during zeolite collapse emerges from small angle scatteringexperiments. Combining powder diffraction with increasing temperature or pressure,we find that the thermobaric characteristics for zeolite collapse have negatived T/d P slopes, consistent with increasing density during amorphization. However, this is not confinedto a single melting curve but, instead, the regime extends over a significant region ofT–P space. Moreover, zeolite amorphization involves depressurization and cavitation effectswhich can be used to set empirical boundaries for the stability of the low densityamorphous phase. Within the region of zeolite instability the pressure or temperature ofamorphization is found to be governed by the rate at which the stress is introduced—themore rapid this is, the higher the pressure or temperature the zeolite structure survives to.The temperature dependence of the rate of collapse is Arrhenian, suggesting that the initiallow density amorphous phase has the characteristics of a superstrong liquid incontrast to the fragility of a conventionally melt quenched glass. Possibilities forcreating ‘perfect glasses’ from the collapse of microporous crystals are discussed.