Temperature-Induced Structural Transformations in Undoped and Eu³⁺-Doped Ruddlesden-Popper Phases Sr₂SnO₄ and Sr₃Sn₂O₇: Relation to the Impedance and Luminescence Behaviors

We report that luminescence of Eu³⁺ ion incorporated into Ruddlesden-Popper phases allows monitoring phase transition in powders (instead of single crystals), in a time-efficient manner (compared to neutron diffraction), and importantly, with greater sensitivity than previous methods. Crystal structure and dielectric response of undoped and 0.5%Eu³⁺-doped Sr₃Sn₂O₇ ceramics were studied as a function of temperature over the temperature range of 300-800 K. The luminescence studies of 0.5%Eu³⁺-doped Sr₂SnO₄ and Sr₃Sn₂O₇ samples were performed in the temperature range of 80-500 K. These results were compared with the respective dependences for the undoped compounds. The structural transformations in 0.5%Eu³⁺-doped Sr₃Sn₂O₇ were found at 390 and 740 K. The former is associated with the isostructural atomic rearrangement that resulted in a negative thermal expansion along two of three orthorhombic crystallographic axes, while the latter corresponds to the structural transition from the orthorhombic Amam phase to the tetragonal I4/mmm one. A similar temperature behavior with the structural transformations in the same temperature ranges was observed in undoped Sr₃Sn₂O₇, although the values of lattice parameters of the Eu³⁺-doped and undoped compounds were found to be slightly different indicating an incorporation of europium in the crystal lattice. A dielectric anomaly associated with a structural phase transition was observed in Sr₃Sn₂O₇ at 390 K. Optical measurements performed over a wide temperature range demonstrated a clear correlation between structural transformations in Eu³⁺-doped Sr₂SnO₄ and Sr₃Sn₂O₇ and the temperature anomalies of their luminescence spectra, suggesting the efficacy of this method for the determination of subtle phase transformations.