Appearance of new photoluminescence peak and spectral evolution of Eu3+ in La2Zr2O7 nanoparticles at high pressure

Abstract

Lots of efforts have been vested recently on tuning the structure and optical properties, particularly for the luminescent nanophosphors. High pressure in that direction has shown great potential to tune the performance of nanophosphors. However, very little attention has been paid on tuning the optical properties of technologically important La2Zr2O7 (LZO) pyrochlore nanoparticles (NPs) and its doped counterpart La2Zr2O7:Eu3+ (LZOE) NPs under elevated pressure. Equally little effort has been given on local structure evolution of Eu3+ ion as a function of pressure in the doped LZOE NPs. Here in this work, LZO depicted strong oxygen vacancy blue emission under ambient and up to 5.54 GPa pressure, and beyond that, there is an evolution of pressure induced green and red photoluminescence (PL) band. This is projected in color tuning from blue → green → white in low-intermediate-high pressure regime. Emergence of unusual visible PL band at elevated pressure is ascribed to structural phase transition from ordered pyrochlore to cotuunite structure which endows large disordering in the LZO structure as well as induces lattice distortion of ZrO6 octahedra. High-pressure PL excitation spectra of the LZOE NPs throw interesting results wherein the charge transfer band is retained in ordered pyrochlore and disappears in high pressure cotuunite phase. As the pressure is raised, the asymmetry ratio of the LZOE NPs exhibits monotonic decrease up to ~19.77 GPa and saturation beyond that, implying a high local symmetry around Eu3+ ions from high pressure compression. This work is a way forward in the area of understanding structure-property correlation, pressure induced color tunability and modulating local structure of lanthanide dopant in the area of luminescent NPs.