Vacuum-Induced Reversible Phase Transformation of Rare-Earth Difluoroacetates: Insights into Structural Evolution, Kinetics, and Luminescence Response

Rare-earth difluoroacetate Eu_1-<i>x</i>Tb_<i>x</i>(dfa)₃(H₂O)₂·H₂O (dfa = CF₂HCOO) exhibits a pressure-dependent crystalline-to-semicrystalline phase transformation similar to that reported for the trifluoroacetate series. This transformation occurs when pressure is decreased below 100 mTorr and is driven by removal of crystallization water and aquo ligands; both types of water molecules participate in hydrogen-bond networks sustaining long-range order. Vacuum-induced dehydration leads to partial amorphization and enhances Tb-to-Eu energy transfer, thus resulting in pressure-sensitive luminescence emission. Rehydration and recrystallization occur upon re-exposure to ambient conditions, indicating that the phase transformation is reversible. Luminescence and powder X-ray diffraction studies show that the kinetics of the atmospheric-to-vacuum and vacuum-to-atmospheric transformations depends on sample matrix and relative humidity. Transformation rates are increased by diluting the difluoroacetate solid in an optically inert matrix and increasing humidity. Dehydration and rehydration processes exhibit fast and slow regimes, depending on whether crystallization or coordinated water is involved. Regimes dominated by reactions involving crystallization water show a marked dependence of activation energy on reaction extent, likely due to the compositional and structural inhomogeneity of the environment in which these reactions occur. Results from structural, kinetic, and luminescence studies enable delineation of a microscopic picture of the phase transformation.