Identification of hydrated phases in ALPO4-5, and explanation of their effect on the water sorption and dynamics

Abstract

A sequence of phases between structural phases in ALPO4-5, commencing from the 4-coordinated aluminum atoms (tetrahedra) in the bare crystal, up to the formation of 6-coordinated (octahedra), and 5-coordinated (bipyramids) aluminum atoms upon the progressive increase of the sorbate water loading, was identified by means of density functional theory calculations, combined with Rietveld refinement of our measured XRD patterns, and produced a crystallographic information file (CIF) at 313 K and relative humidity 81.5%. We modeled the thermodynamics of the sorbed water at progressively increasing chemical potentials via grand canonical Monte Carlo computations, and explained, on the basis of the formed phases, the hysteresis loop which appears in the gravimetrically measured isotherms of the literature during the sorption-desorption cycles. Molecular dynamics simulations predicted the impact of the existence of hydrated phases on the sorbate water diffusivity; collective diffusivities at the linear response regime were also calculated and thereby, transport coefficients.