Investigation of water assisted phase transformation process from AlPO4-5 to AlPO4-tridymite

• Transformation of AlPO4-5 into AlPO4-tridymite was realized by heating with water.
• In situ multinuclear MAS NMR was used to study the phase transformation.
• Water first activates residue amorphous phase to transform into AlPO4-tridymite.
• Water participates the transformation by hydrolysis and condensation reaction.
• Water establishes the mass transportation between AlPO4-5 and AlPO4-tridymite.

Water assisted phase transformation process from crystalized AlPO4-5 to AlPO4-tridymite was studied by the combination of X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and in situ multinuclear MAS NMR. It is found that water first activates the residue amorphous aluminophosphate in crystalized AlPO4-5 sample through hydrolysis and condensation reactions. Then the activated aluminophosphate species reassemble into AlPO4-tridymite crystalline. Meanwhile, AlPO4-5 transforms into orthorhombic phase during heating process. With further crystallization of AlPO4-tridymite, the amorphous phase is gradually consumed, and mass transportation between AlPO4-5 and AlPO4-tridymite is established through gradually amorphization of AlPO4-5. Finally, most of the AlPO4-5 transforms into the thermodynamically stable dense phase AlPO4-tridymite.

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