Tunable optical emissions of Eu3+ ions enabled by pressure-driven phase transition in ZnO

Controlling the optical properties of rare-earth ions in wide-bandgap semiconductors remains a major challenge in the development of next-generation photonic materials. Here, we show that external hydrostatic pressure modulates the structural characteristics of ZnO thin films and, in turn, tunes the optical emission behavior of embedded Eu3+ ions. By combining in situ synchrotron X-ray diffraction and photoluminescence spectroscopy under high-pressure conditions with first-principles calculations, we capture a pressure-induced phase transition from the hexagonal wurtzite to the cubic rocksalt structure near 10 GPa. This transformation is accompanied by complete quenching of the D0 - FJ Europium emissions near the transition threshold, followed by a partial recovery at higher pressures, likely associated with the emergence of structural disorder. Concurrently, the Stark components of the emission bands exhibit a redshift and significant broadening with increasing pressure, reflecting enhanced crystal field strength as interatomic distances decrease. Additional first-principles calculations support the observed pressure-induced shifts in the Eu-4f states and emphasize the influence of lattice symmetry on their electronic environment. These results show that hydrostatic pressure is an effective way to adjust the optical emissions of rare-earth ions by changing their symmetry and local environment, providing a basis for designing photonic devices and luminescent materials controlled by pressure.