Amorphous Silica Obtained from the Zeolite Chabazite due to Structural Collapse under Mild Pressure-Temperature Conditions.

The siliceous zeolite chabazite, which has a very large void volume for a crystalline silica polymorph, was found by in situ synchrotron, X-ray diffraction in diamond anvils cells to collapse and become amorphous under mild pressure-temperature conditions in the ranges of 0.5-1 GPa and 320-454 K. This procedure was scaled up under technologically relevant conditions of around 0.7 GPa and 473 K in a standard pellet pressing die equipped with a band heater. The materials obtained are distinct from silica glass with first sharp diffraction peaks at close to 1.60-1.63 Å-1, shorter intertetrahedral distances, and different infrared and 29Si NMR spectra. The mechanism for this collapse is dominated by the closure of the 8-membered rings of SiO4 tetrahedra in the large chabazite cages in the structure, resulting in close to a 40% decrease in volume almost entirely due to the reduction of the c lattice parameter of rhombohedral chabazite. These results open the way to obtain novel amorphous forms of silica with characteristic medium-range order under mild P-T conditions, which could be expected to have distinct optical and mechanical properties with respect to silica glass.