(376b) The Interrelationship between Lanthanide and Actinide Dopants and the Local Environments on the Luminescent Properties of Complex Metal Oxide

Rare earth (RE) based phosphor materials are widely used as luminescent probes in high temperature optical thermometry where standard methods are unsuitable. Currently, the luminescent efficiency and sensitivity of these temperature dependent phosphors are poor, requiring the development of alloyed materials with favorable vibrational coupling at desired temperatures. The current generation of luminescent thermometers, such as Y stabilized ZrO₂ and NaYF₄, lack stability and energy transport within the working temperature ranges. As such, the challenge is to develop an oxide-based material with high luminescence intensity and thermal sensitivity/stability, which is suitable for an optical temperature sensor/thermal barrier coating. The goal for this work is to develop a fundamental understanding of the local environment surrounding RE or actinide dopants in metal oxide hosts to engineer their luminescence property. To maximize the luminescence, local symmetry, doping concentration, and radiative/non-radiative relaxation pathways need to be simultaneously optimized. This interaction between the dopant ions and host matrices, and the resulting impact on the luminescent signal, is the focus of this work and is critical for engineering the application dependent luminescent properties.

Herein, a two-step synthesis method of co-precipitation and molten salt was employed to prepare complex RE-doped metal oxides. For the first step, Tb doped Y₂Zr₂O₇ (YZO) was applied to study the Ce sensitizing effect to the local structure and luminescence intensity. The single doped YZO:Tb (2 mol%) NPs shows a strong Tb³⁺ emission. However, after co-doping with Ce ions, the Tb³⁺ emission is quenched instead of the expected sensitization and was attributed to Ce driven oxidization to a non-luminescent Tb⁴⁺ state. This was then extended to multivalent systems, such as uranium ions, in La₂Hf₂O₇ (LHO) and Gd₂Hf₂O₇ (GHO) host crystals to study the effect of structure on the dopant state. A U-driven phase transfer in LHO and GHO were observed to stabilize the local coordination environment surround the dopants, resulting in two distinct U clusters based on the available lattice site size. This response was also used to enhance the Er upconversion luminescence by co-doping the host with smaller cations (Sc³⁺). Increasing the Sc concentration in Y_2-x Sc_xO₃ nanoparticles allows us to constrict the C₂ RE-O bond distance to be more favorable for Er incorporation compared to the low PL efficiency C_3i sites. PL results indicated the composition of Y_1.75Sc_0.25O₃:5% Er has the most intense both upconversion and downconversion responses. Judd-Ofelt calculation showed that YScO:Er with Sc concentration less than 50% of (Y+Sc) have improved efficiency and intensity, and decreased site symmetry level, indicating the incorporation of small amount Sc drives Er ions to occupy lower symmetry sites (C₂) and reduces the overall site symmetry. Finally, these special and structure controls of RE dopants were applied in designing a qualifying material for high temperature application such as thermal barrier coating. This work offers an idea about the relation between dopants and the structure-property change of different host materials, which can be applied into designing materials for different applications such as phosphors, solid state lasers and temperature sensors.