Crystalline phase of Y 2O 3:Eu particles generated in a substrate-free flame process

Abstract

In this study, factors affecting the crystal structure of flame-synthesized Y 2O 3:Eu particles were investigated, especially the particle size effect and its interaction with Eu doping concentration. Polydisperse Y 2O 3:Eu (size range 200 nm to 3 μm) powder samples with Eu doping concentrations from 2.5 mol% to 25 mol% were generated in either H 2/air or H 2/O 2 substrate-free flames. The crystal structure of the powder samples was determined by powder X-ray diffraction (XRD), which was complemented by photoluminescence (PL) measurements. Single particle crystal structure was determined by single particle selected area electron diffraction (SAED), and for the first time, by electron backscatter diffraction (EBSD). H 2/air flames resulted in cubic phase Y 2O 3:Eu particles with hollow morphology and irregular shapes. Particles from H 2/O 2 flames had dense and spherical morphology; samples with lower Eu doping concentrations had mixed cubic/monoclinic phases; samples with the highest Eu doping concentrations were phase-pure monoclinic. For samples generated from H 2/O 2 flames, a particle size effect and its interaction with Eu doping concentration were found: particles smaller than a critical diameter had the monoclinic phase, and this critical diameter increased with increasing Eu doping concentration. These findings suggest that the formation of monoclinic Y 2O 3:Eu is inevitable when extremely hot substrate-free flames are used, because typical flame-synthesized Y 2O 3:Eu particle sizes are well below the critical diameter. However, it may be possible to generate particles with dense, spherical morphology and the desired cubic structure by using a moderately high flame temperature that enables fast sintering without melting the particles.