Sm³⁺ doped calcium orthovanadate Ca₃(VO₄)₂ - A spectral study

Sm³⁺-doped vanadate-based phosphors, Ca_3-x (VO₄)₂:Sm_x(x = 0.005 ≤ x ≤ 0.095), were synthesized by the citrate-based sol-gel method. The samples were characterized with XRD, SEM, diffuse reflectance and photoluminescence spectroscopy, aiming at the development of phosphor-converted wLED applications. Using diffuse reflectance spectra and the Kubelka-Munk function, the band gap energy was calculated to be 3.28 eV for the Ca_2.95(VO₄)₂:Sm_0.05 phosphor. Observation of the photoluminescence excitation spectra revealed a broadband excitation from the distorted VO₄ tetrahedron and a few narrow peaks from the 4f-4f intra-configuration transitions of the Sm³⁺ ions. Upon 404 nm excitation, ⁴G_5/2 → ⁶H_5/2 (564 nm), ⁴G_5/2 → ⁶H_7/2 (601 nm), ⁴G_5/2 → ⁶H_9/2 (648 nm) and ⁴G_5/2 → ⁶H_11/2 (701 nm) transitions were observed. The optimum Sm³⁺ concentration for the studied phosphor is 0.05 mol, and the estimated critical distance between Sm³⁺ ions (R_c) is 19.1 Å. The observed quenching of Sm³⁺ emission is attributed to the dipole-dipole interactions. The possible cross-relaxation process between neighboring Sm³⁺ ions was discussed in detail. The CIE coordinates were calculated for all the samples, and they set on the orange region of CIE diagram. In addition, pure orange color emission was observed under 365 nm UV lamp. The results suggest that the Ca₃(VO₄)₂:Sm³⁺ phosphor could be a good candidate for the orange emitting component for wLED applications.