Crystal structure of blue–white–yellow color-tunable Ca4Si2O7F2:Eu2+,Mn2+ phosphor and investigation of color tunability through energy transfer for single-phase white-light near-ultraviolet LEDs

Abstract

The crystal structure of Ca₄Si₂O₇F₂:Eu²⁺,Mn²⁺ was refined and determined from X-ray diffraction (XRD) profiles obtained using a synchrotron light source by the Rietveld refinement method. It was found to crystallize into a monoclinic structure with the P2₁/c(14) space group. On examining the Mn²⁺-concentration-dependent photoluminescence properties, we found that the emission colors could be tuned from blue (0.152, 0.112) to white-light (0.351, 0.332) and eventually to yellow (0.430, 0.423) through energy transfer by changing the Eu²⁺/Mn²⁺ ratio. Moreover, energy transfer from a sensitizer Eu²⁺ to an activator Mn²⁺ occurs via a resonance-type dipole–quadrupole interaction mechanism, and the critical distances of the energy transfer were calculated to be 11.66 Å and 12.61 Å using concentration quenching and spectral overlap methods, respectively. Combining a 400 nm near-ultraviolet (NUV) chip and a single-phase white-emitting (Ca_0.96Eu_0.01Mn_0.03)₄Si₂O₇F₂ phosphor produced a white-light NUV LED with CIE chromaticity coordinates of (0.347, 0.338) and a warm color temperature of 4880 K.