Strongly Modified Mechanical Properties and Phase Transition in AlPO(4)-17 Due to Insertion of Guest Species at High Pressure

[Image: see text] The porous aluminophosphate AlPO(4)-17 with a hexagonal erionite structure, exhibiting very strong negative thermal expansion, anomalous compressibility, and pressure-induced amorphization, was studied at high pressure by single-crystal and powder X-ray diffraction in the penetrating pressure transmitting media N(2), O(2), and Ar. Under pressure, these guest species were confirmed to enter the pores of AlPO(4)-17, thus completely modifying its behavior. Pressure-induced collapse in the xy plane of AlPO(4)-17 no longer occurred, and this plane exhibited close to zero area compressibility. Pressure-induced amorphization was also suppressed as the elastic instability in the xy plane was removed. Crystal structure refinements at a pressure of 5.5 GPa indicate that up to 28 guest molecules are inserted per unit cell and that this insertion is responsible for the reduced compressibility observed at high pressure. A phase transition to a new hexagonal structure with cell doubling along the a direction was observed above 4.4 GPa in fluid O(2).