Observation and explanation of the reversible pressure-induced amorphization of Ca(NO3)2/NaNO3

X-ray and Raman measurements show that crystalline Ca(NO3)2/NaNO3 becomes amorphous above a pressure of 9.4 GPa. Upon release of the pressure, the crystalline state returns. We report reversible pressure-induced amorphization for a substance that can be prepared as a melt-quenched glass at ambient pressure. We suggest a mechanism for the amorphization based on the formation of a noncrystallographic packing of polytetrahedral units. A consideration of several pressure-induced amorphizations indicates that polytetrahedral amorphization mechanisms are appropriate for nondirectionally bonded materials, but are not appropriate for solids where covalent and hydrogen bonding predominates. Polytetrahedral ordering provides a link between amorphous states formed by compression and those formed by more conventional methods at ambient pressure (e.g., melt quenching).