Ca₉Gd(PO₄)₇:Sm³⁺—a novel single-phased down converting orange-red-emitting nanophosphor

Nanocrystalline orange-red-emitting white voluminous powdered samples of Sm³⁺-doped Ca₉Gd(PO₄)₇ phosphor (CGPS) were synthesized by highly efficient solution combustion route. Highly advanced characterization techniques like X-ray diffraction (XRD), Transmission electron microscopy (TEM), and Photoluminescence (PL) spectroscopy were used to characterize the series of samples systematically. The structural properties of Ca₉Gd_0.88(PO₄)₇:0.12Sm³⁺ (best emitting composition) phosphor were examined in detail via Rietveld refinement analysis to get the information about its structural prototype and crystal phase purity. Phase analysis results of the best emitting composition revealed that the introduction of dopant (Sm³⁺) ion into the host matrix did not induce any modification in the crystal structure and it further confirmed the crystallization of the Ca₉Gd_0.88(PO₄)₇:0.12Sm³⁺ phosphor in a trigonal lattice with R3c(161) space group. With the help of the Scherrer’s equation, the average crystallite size of Ca₉Gd_0.88Sm_0.12(PO₄)₇ system was found to be 40–50 nm. The PL investigations resulted that the optimum Sm³⁺ ion concentration in Ca₉Gd(PO₄)₇ host matrix was 12 mol%. Diffuse reflectance (DR) measurements gave the band-gap value of 4.16 eV for Ca₉Gd_0.88Sm_0.12(PO₄)₇ nanophosphor. Detailed PL studies of CGPS phosphors indicated that with the excitation wavelength of 403 nm there was an intense orange-red emission appearing at 600 nm due to the partially electric and partially magnetic dipole allowed ⁴G_5/2 → ⁶H_7/2 transition. The critical energy transfer distance (R_c) was calculated in accordance with Van Uitert model which came out to be 21 Å and the proposed mechanism behind energy transfer between different Sm³⁺ ions was analyzed to be dipole–quadrupole interactions using Dexter’s analysis. In addition to the radiative lifetime (2.4 ms), non-radiative transition rate (142.9 s⁻¹) and the value of quantum efficiency (75%) were also analyzed with the help of Auzel’s fit function. The results as-obtained favor the potential utility of these promising nanophosphors in the domain of lighting for white LEDs.