Extreme deformation mechanisms in open-framework silicates at high-pressure : evidence of anomalous inter-tetrahedral angles

This is a comparative study on the flexibility of tetrahedral open-framework structures (i.e. zeolites and zeolites-like materials) at high-pressure (HP). Analysis of the main P-induced deformation mechanisms that force the inter-tetrahedral (T–O–T) angles toward values drastically smaller than 120°, is carried out on the basis of recent data obtained by in situ HP-diffraction experiments and theoretical calculations. The role played by the nature of the framework and extra-framework cations in isotypic structures on the framework flexibility is discussed. A comparative analysis between the structural evolution of some structures that show anomalously small T–O–T angles at high-pressure and their structural configuration at high-temperature is carried out. Tetrahedral framework silicates react in response of the applied pressure in different ways, here discussed, toward structural configurations energetically costly, with T–O–T angle 120°, but maintaining their topological symmetry up to the onset of the amorphization processes. Reconstructive phase-transitions, with a change in topology, do not occur in this class of materials.