Strongly Modified Mechanical Properties and Phase Transition in AlPO₄-17 Due to Insertion of Guest Species at High Pressure

Abstract

The porous aluminophosphate AlPO 4 -17 with a hexagonal erionite structure, exhibiting very strong negative thermal expansion, anomalous compressibility, and pressureinduced 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 .

Direct Single Crystal to Amorphous Transformation and Memory Effect in AlPO4-17

“…Additionally, in the remaining crystalline phase above 0.9 GPa a large number of superlattice hkl reflections (Figure S4) with half integer indices in h and k (i.e., 1/2 0 8, 3/2 0 2, 3/2 5/2 4, etc.) with a maximum relative intensity of 4% appear indicating cell doubling along a and b as has also been found in oxygen filled AlPO 4 -17 at much higher pressure . No clear distortion from a hexagonal unit cell metric is observed and any eventual lowering in symmetry could not be reliably determined due to the reduction in data quality at this pressure.…”