Crystalline-to-semicrystalline transition in lanthanide trifluoroacetates: implications for optical pressure and temperature sensing

Abstract

We report the serendipitous discovery of a pressure-dependent, crystalline-to-semicrystalline, reversible transition in the solid solution Eu_1−xTb_x(tfa)₃(H₂O)₃ (tfa = CF₃COO). This transition is observed across the entire composition range upon reducing pressure from atmospheric to sub-100 mTorr levels. In the case of mixed-metal trifluoroacetates, switching from the atmospheric to the low-pressure phase leads to a drastic reduction in the intensity of green emission from Tb³⁺ and enhanced red emission from Eu³⁺. The low-pressure phase shows not only enhanced Tb-to-Eu energy transfer but also a multisite distribution of lanthanide emitters. The crystalline-to-semicrystalline transition results from a disruption of hydrogen bonds and van der Waals interactions responsible for the long-range order observed in the atmospheric phase, likely driven by chemical changes such as vacuum-induced removal of terminal acuo ligands. The distinct luminescence responses of the atmospheric and low-pressure phases of Eu_1−xTb_x(tfa)₃(H₂O)₃ have direct implications for the potential of mixed-metal trifluoroacetates as optical sensors. On the one hand, this contrast could be exploited to realize a two-state luminescent pressure sensor, though with the limitations imposed by the kinetics of the pressure-dependent transition, which takes about a day to complete in either direction under the experimental conditions observed in this work. On the other hand, the much weaker intensity of the emission from Tb³⁺ relative to that of Eu³⁺ in the low-pressure phase (≈2 orders of magnitude) limits the robustness of the red-to-green ratio as a thermometric parameter for luminescent thermometry.