Crystallographic phase-driven tuning of Eu3+ emission in Gd2Ge2O7 ceramics for potential optical applications

We report a comprehensive study on the phase-dependent luminescence of Eu3+-doped Gd2Ge2O7 pyrogermanates, highlighting the unique role of Eu3+ ions as structural and optical probes for distinguishing tetragonal and triclinic polymorphs. By tailoring the synthesis temperature (1100–1300 °C), we achieved precise control over the crystalline phase, which was directly reflected in the emission features of Eu3+. Eu3+ emission bands were observed not only from the 5D0 level but also from higher-energy states (5D1, 5D2 and 5D3), with significantly sharper features in the tetragonal phase. Excitation at 230 nm proved highly selective for the tetragonal phase, while the emission spectra exhibited strong excitation-wavelength dependence, with orange-red emission at 275 nm and a shift toward blue under 394 nm excitation. The emission decay profiles of the 5D0 state were also analyzed, revealing lifetimes of approximately 1.6 ms for the tetragonal phase and 1.9 ms for the triclinic phase. Time-resolved emission analysis indicated that emissions from higher excited states could be observed with delays up to 1.0 ms, while those from 5D0 were noticeable with a delay of 7.0 ms. Solid solution studies in Eu2-xGdxGe2O7 (x = 0.01–1.99) demonstrated that Eu3+ emissions characteristic of the tetragonal phase remained observable up to x = 1.60, aligning with structural analyses. These results underscore the intrinsically low phonon energy of the pyrogermanate host and provide valuable insights into structure–luminescence correlations in rare-earth-doped ceramics, establishing a foundation for the rational design of phase-selective phosphors for photonic applications.