Governing the crystallographic sites for tuning Eu 2+ emission in an apatite oxyfluoride host to be applied for superior white light emitting diodes.

Single white light-emitting phosphors for near-UV-converted white light-emitting diodes (WLEDs) are the best alternatives to tricolour phosphor blends and blue light converted yellow-emitting garnets. Nevertheless, achieving white light with elevated colour rendering (CRI) from a single-phase phosphor activated with a lone activator ion is a major challenge. This study aimed at the generation of white light from a single-phase composition activated only with europium. The study started with structural evaluations of Eu 3+ -activated Ca 4 La 6 Si 6 O 24 F 2 phosphors using X-ray diffraction (XRD) and Eu 3+ photoluminescence to elucidate the local environment of rare-earth ions and the symmetric nature of the lattice sites. Ca 4 La 6 Si 6 O 24 F 2 crystallized in the hexagonal P 6 3 / m space group. The predominant 5 D 0 - 7 F 2 electric dipole transition at 614 nm, and the non-splitting as well as the zero-shifting behaviour of 5 D 0 - 7 F 0 at 578 nm, suggested that the rare-earth ionic substitutions preferably took place at the larger asymmetric sites. Introducing Sr 2+ ions in Ca 4 La 6 Si 6 O 24 F 2 :Eu 3+ /Eu 2+ that is synthesized under a reducing atmosphere suppressed Eu 3+ emission. From the optimized Ca 1.98 Sr 1.98 La 6 Si 6 O 24 F 2 :0.04Eu 2+ , a sequence of M 2+ -codoped (M = Mg/Ba) Ca 1.98 Sr 1.98 La 6 Si 6 O 24 F 2 :0.04Eu 2+ phosphors were further developed. The substitutions of Mg 2+ and Ba 2+ altered the crystal field by changing the lattice parameters. The Mg 2+ -doped samples showed a blue-shift from 520 nm (Mg 2+ = 0) to 471 nm (Mg 2+ = 1.0), whereas the Ba 2+ -doped compositions showed a red-shift from 520 nm (Ba 2+ = 0) to 536 nm (Ba 2+ = 1.2). The change of symmetry owing to the Mg 2+ /Ba 2+ substitution could have led to the centroid shift, which was responsible for the blue- or red-shift of the emission spectra. The XRD of Ca 1.38 Sr 1.38 La 6 Si 6 O 24 F 2 :0.04Eu 2+ ,1.2Ba 2+ indicated a Ba 2+ -induced lattice site expansion. Keeping this in view, the Eu 2+ ions concentrations were further enhanced from 0.04 to 0.3, and the resultant photoluminescence was further enhanced and red-shifted. The optimized sample showed better intensity compared with the commercial Y 3 Al 5 O 12 :Ce 3+ and exhibited decent photoluminescence above 70% at 150 °C as compared with that at room temperature. Finally, several prototype WLEDs were fabricated using the single phosphor Ca 1.365 Sr 1.365 La 6 Si 6 O 24 F 2 :0.07Eu 2+ ,1.2Ba 2+ with near-UV and violet-LED chips. The outcomes indicated the promising nature of this single-composition phosphor for indoor lighting.