Governing the crystallographic sites for tuning Eu2+ emission in an apatite oxyfluoride host to be applied for superior white light emitting diodes.

Abstract

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 Eu3+-activated Ca4La6Si6O24F2 phosphors using X-ray diffraction (XRD) and Eu3+ photoluminescence to elucidate the local environment of rare-earth ions and the symmetric nature of the lattice sites. Ca4La6Si6O24F2 crystallized in the hexagonal P63/m space group. The predominant 5D0-7F2 electric dipole transition at 614 nm, and the non-splitting as well as the zero-shifting behaviour of 5D0-7F0 at 578 nm, suggested that the rare-earth ionic substitutions preferably took place at the larger asymmetric sites. Introducing Sr2+ ions in Ca4La6Si6O24F2:Eu3+/Eu2+ that is synthesized under a reducing atmosphere suppressed Eu3+ emission. From the optimized Ca1.98Sr1.98La6Si6O24F2:0.04Eu2+, a sequence of M2+-codoped (M = Mg/Ba) Ca1.98Sr1.98La6Si6O24F2:0.04Eu2+ phosphors were further developed. The substitutions of Mg2+ and Ba2+ altered the crystal field by changing the lattice parameters. The Mg2+ -doped samples showed a blue-shift from 520 nm (Mg2+ = 0) to 471 nm (Mg2+ = 1.0), whereas the Ba2+-doped compositions showed a red-shift from 520 nm (Ba2+ = 0) to 536 nm (Ba2+ = 1.2). The change of symmetry owing to the Mg2+/Ba2+ substitution could have led to the centroid shift, which was responsible for the blue- or red-shift of the emission spectra. The XRD of Ca1.38Sr1.38La6Si6O24F2:0.04Eu2+,1.2Ba2+ indicated a Ba2+-induced lattice site expansion. Keeping this in view, the Eu2+ 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 Y3Al5O12:Ce3+ 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 Ca1.365Sr1.365La6Si6O24F2:0.07Eu2+,1.2Ba2+ with near-UV and violet-LED chips. The outcomes indicated the promising nature of this single-composition phosphor for indoor lighting.