AlPO₄-54–AlPO₄-8 Structural Phase Transition and Amorphization under High Pressure

Abstract

Microporous AlPO4-54, which exhibits the largest pores among zeolites and aluminophosphates with a diameter of 12.7 Å, was investigated at high pressure by X-ray powder diffraction (XRD), mid- and far-infrared (IR) spectroscopy in diamond anvil cells. The material undergoes a phase transition beginning around 0.8 GPa. The amount of AlPO4-8 gradually increases with pressure and the phase transition is complete between 2 and 3 GPa. The closure of the (POAl) angle destabilizes the structure of AlPO4-54, which drives the transition to AlPO4-8. The pressure-induced phase transformation of AlPO4-54 to AlPO4-8 is associated with a symmetry reduction from hexagonal to orthorhombic and with a change in the unidirectional ring channel parallel to the c-axes from 18 to 14 AlO4 and PO4 tetrahedra. An abrupt decrease along the b direction is linked to the formation of 4 new rings of 6 tetrahedra with significant structural reorganization. The transition is followed by irreversible amorphization beginning around 3.5 GPa due to the collapse of the pores. The amorphization of AlPO4-8 was detected from the disappearance of the XRD lines, abrupt shifts, and strong broadening of the mid-IR modes, and by changes in the pressure dependence of the mid- and far-IR modes, indicating a lower compressibility for the more dense amorphous form. Far-IR spectra indicate that the new amorphous form retains the local structure of AlPO4-8.

An in-situ x-ray diffraction and infrared spectroscopic study of the dehydration of AlPO4-54

“…An alternative method using polyphosphoric acid was used for the synthesis of the large single crystals [16]. The details of the synthesis for both samples can be found elsewhere [4,15,17]. The polycrystalline material was ground prior to the in situ studies under vacuum.…”

“…In addition, unit cell parameters differ depending on the vacuum pressure, temperature and duration of the treatment, for example after 8h at 1.3 mPa the unit cell parameters were found to be a=18.524 Å and c = 8.332 Å [10] as compared to a=18.544 Å and c = 8.3847 Å after 48h at 0.1 Pa [11], with no further evolution under high vacuum nor with heating. Very long dehydration with increasing vacuum and temperature over the period of one week produced a hexagonal material with a=18.5457 Å and c = 8.3992 Å [15].…”

Effect of H2O on the Pressure-Induced Amorphization of Hydrated AlPO4-17

“…AlPO 4 -54• x H 2 O possesses a 1-D pore system along the c direction, in which H 2 O molecules form a disordered hydrogen-bonded network [11]. While its dehydrated phase undergoes to pressure-induced phase transition starting at 0.8 GPa, its hydrated phase shows PIA beginning at 2.0 GPa [12,13]. In H 2 O, the material undergoes superhydration effects and a decrease in the onset of amorphization from 2.0 in non-penetrating PTM to 0.9 GPa in penetrating H 2 O PTM is observed [12,13].…”

“…While its dehydrated phase undergoes to pressure-induced phase transition starting at 0.8 GPa, its hydrated phase shows PIA beginning at 2.0 GPa [12,13]. In H 2 O, the material undergoes superhydration effects and a decrease in the onset of amorphization from 2.0 in non-penetrating PTM to 0.9 GPa in penetrating H 2 O PTM is observed [12,13]. The insertion of H 2 O in the pores hinders pore collapse at lower pressures.…”

Structure and mobility of water confined in AlPO4-54 nanotubes

“…AlPO 4 -54 possesses unidirectional channels parallel to the z-axis formed by 18 AlO 4 and PO 4 tetrahedra with all Al and P cations in 4-fold coordination. 3,4 Our motivation to study water confined within AlPO 4 -54 nanotubes is due to the potential application of this material as nanosieve for water treatment. Specifically, the size of the AlPO 4 -54 pores is large enough to allow for the flow of water molecules, while small enough to filter small organic molecules.…”