Structural and Photoluminescence Characterization of Oxyfluorides for Phosphor Applications

Abstract

ConspectusRare-earth-containing phosphors were crucial to the advances made to compact fluorescent lamps (CFLs), which assisted in protecting a widely used halophosphate phosphor from degrading after exposure to a high ultraviolet flux. The CFL phosphors are often coated twice by depositing a light coat of rare-earth-containing phosphors over the inexpensive halophosphate phosphor, which generates white light with high efficacy and a good color rendering index and possesses a balance between phosphor cost and performance. Costs of phosphors can be mitigated by requiring lower rare-earth ion concentrations or by completely eliminating rare-earth ions, which was one of the main goals of investigating the oxyfluorides Sr3AlO4F and Ba2SrGaO4F as potential phosphors. Changes in the Sr3AlO4F and Ba2SrGaO4F structures were studied using high-resolution neutron diffraction annealing these materials in 5%H2/95%Ar and 4%H2/96% Ar, respectively. Annealing in these atmospheres causes self-activated photoluminescence (PL) to occur under 254 nm light, which makes them ideal materials for rare-earth-free CFL phosphors. Additionally, these hosts possess two distinct sites for isovalent or aliovalent substitution of Sr denoted as the A(1) and A(2) sites. Ga3+ can be substituted for Al3+ at the M site, which is known to have an impact on the self-activated PL emission color. The structural distortions noted included closer packing in the FSr6 octahedrons and AlO4 tetrahedrons in the Sr3AlO4F structure as compared to in air-annealed samples, which show no PL emission. Temperature-dependent studies reveal that both the air- and reductively annealed samples have identical thermal expansion within this temperature range (3-350 K). High-resolution neutron diffraction at room temperature confirmed the tetragonal structure (I4/mcm) for Ba2SrGaO4F, a novel material in the Sr3AlO4F family of materials, has been synthesized via a solid-state method. Analysis of the refined Ba2SrGaO4F structure at room temperature revealed expansion in the lattice parameters and its polyhedral subunits between the reductively annealed and air-annealed samples, which are correlated with the PL emission. Previous studies related to the application of these host structure types revealed that they have potential as commercial solid-state lighting phosphors due to their ability to resist thermal quenching as well as accommodate various levels of substitutions that will assist with color tunability.