An in situ time-of-flight neutron powder diffraction study of the humidity-induced phase transition in sodium monothiophosphate

Anhydrous β-Na3PO3S, obtained by rapid cooling, has been observed to convert to the more thermodynamically stable, anhydrous α-Na3PO3S upon exposure to a humid atmosphere. Though water plays a critical role in this polymorphic transformation, it is absent in both the initial and final materials. In situ time-of-flight neutron powder diffraction was used to track the progress of this transition as a function of time. Diffraction data were acquired using the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory. An empirical whole pattern fitting method was used to determine the polymorphic phase composition over time. The resulting data were evaluated using several solid-state kinetic models. The kinetics of this transition has been determined to be best described by either one- or two-dimensional diffusion. The significance of these models with respect to the physical phenomenon is discussed.