Ca6La4(SiO4)2(PO4)4O2:Eu2+: a novel apatite green-emitting phosphor for near-ultraviolet excited w-LEDs

Abstract

A novel apatite phosphor Ca₆La₄(SiO₄)₂(PO₄)₄O₂:Eu²⁺ was prepared by conventional high-temperature solid-state reaction. Phase purity was examined by XRD and XPS analysis. The crystal structure information, such as space group, cell parameters and atomic coordinates, were refined by the Rietveld method, revealing that Eu²⁺ occupied the sites of Ca²⁺ ions. Moreover, low-temperature experiments, including low-temperature PL spectra and low-temperature decay curve, were used to prove the existence of two luminescence centers in Ca₆La₄(SiO₄)₂(PO₄)₄O₂:Eu²⁺. With the increase in doping concentration of Eu²⁺, the emission wavelength shows a red shift from 498 nm to 510 nm, which is mainly caused by the increase in crystal-field splitting by Eu²⁺. The optimized concentration of Eu²⁺ was confirmed to be 0.01, the R_c was calculated to be 20.09 Å and the energy transfer between Eu²⁺ was demonstrated to be by exchange interaction. Moreover, good thermal stability has been proved by a temperature-dependence experiment; it shows that the phosphor can maintain 55% of emitting intensity at 150 °C compared to that at room temperature. Finally, the Ca₆La₄(SiO₄)₂(PO₄)₄O₂:Eu²⁺ phosphor was fabricated with commercial red (CaAlSiN₃:Eu²⁺) and blue (BAM:Eu²⁺) phosphor coating on a n-UV chip. This proves that this green phosphor has the potential to be used in a w-LED lamp.