High pressure induced local ordering and tunable luminescence of La2Hf2O7:Eu3+ nanoparticles

Abstract

High pressure spectroscopy is of immense importance in understanding materials with respect to their high pressure phases, structural phase transitions, designing pressure sensors, etc. With this objective in mind, we have carried out time resolved photoluminescence and Raman spectroscopy on La₂Hf₂O₇:Eu³⁺ (LHOE) nanoparticles (NPs) under applied high pressure. High pressure Raman spectra of the LHOE NPs suggest enhanced disordering at high pressure and shifting of the most intense F_2g Raman band to higher wavenumber. High pressure excitation photoluminescence spectra of the LHOE NPs demonstrate that the O²⁻ → Eu³⁺ charge transfer band quenches beyond 2.7 GPa whereas not many changes could be observed from the intra-configurational f–f excitation bands of Eu³⁺ ion. High pressure emission photoluminescence spectra of the LHOE NPs show gradual lowering of asymmetry ratio value, reduced Stark component in magnetic dipole transition, and the disappearance of the ⁵D₀ → ⁷F₀ transition of Eu³⁺ ion at high pressure. These observations suggest significant improvement in Eu³⁺ symmetry. These results are further corroborated with lifetime spectroscopy, which indicates high pressure induced tunneling of Eu³⁺ ion from La³⁺ to Hf⁴⁺ sites. These changes are visualized by beautiful color tunability of the LHOE NPs from red to orange to yellow and correlated color temperature from 1900 to 2600 K. This work demonstrates a new mode of designing optically tunable luminescent nanomaterials and understanding high pressure induced changes in photonic materials.