Effect of Ca2+ ion co-doping on radiative properties via tuning the local symmetry around the Eu3+ ions in orange red light emitting GdPO4:Eu3+ phosphors

A series of Ca2+ substituted GdPO4:Eu3+ novel phosphors were prepared via the solid state method. Rietveld refinement of the XRD data and transmission electron microscopy results confirmed that all these compounds adopted the monazite phase with space group P21/n. Fourier transform infrared spectroscopy (FTIR) analysis confirmed the presence of the characteristic vibrational bands for host matrix of GdPO4, and field emission scanning electron microscopy (FESEM) results revealed that the particles possess agglomerated granular morphology. Photo-luminescent spectra displayed the representative luminescence D-5(0) F-7(J) (J = 0-4) intra-4f shell Eu3+ ion transitions. The magnetic dipole (D-5(0) F-7(1)) transition was stronger than the electric dipole (D-5(0) F-7(2)) transition. On co-doping Ca2+ content to Gd0.93Eu0.07PO4 phosphor, we observed the enhanced PL intensity as a function of Ca2+ content; the maximum intensity of 1.5 times was observed for 7 mol% of calcium doped Gd0.93Eu0.07PO4 phosphor. The enhancement of the PL intensity owing to the effective ionic radius and the mismatch in Pauling's electronegativity between co-dopant and host cations causes distortion of local crystal field surrounding the Eu3+ ions. Furthermore, this local distortion or reduced symmetry effect around the Eu3+ ions was reconfirmed by the Judd-Ofelt and life time decay analyses. The evaluated 1931 Commission International de l'Eclairage (CIE) chromaticity color coordinates exhibit orange red emission (x = 0.6247, y = 0.3748) with minimal CCT values and high color purity. This class of phosphors possessed excellent thermal stability at high temperature, and the integrated emission intensity at 423 K was about 66% of that at 303 K. Considering the above results, the Eu3+/Ca2+ co-doped GdPO4 phosphors have potential applications in the near-UV excited white light emitting diodes.