Ab-initio DFT calculations and experimental investigations into optoelectronic and structural properties of Ca9Al(PO4)7:Sm3+ orange phosphor

Nanocrystalline phosphors of composition Ca9Al(PO4)7:xSm3+ (x = 0.01–0.16) were successfully synthesized using a urea-assisted highly efficient solution combustion technique. The structural, morphological, and photoluminescence features of the series of white powdered samples were studied using powder X-ray diffraction, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and photoluminescence characterization techniques. The structural framework and crystal phase purity of Ca9Al0.90Sm0.10(PO4)7 phosphor were investigated by Rietveld refinement analysis. The results showed that the replacement of Al3+ ions with Sm3+ ions in Ca9Al0.90Sm0.10(PO4)7 nanophosphor didn't alter the host lattice crystal structure and a trigonal lattice having R3c(161) space group was established. Meanwhile, the average particle size evaluated from the Scherrer equation was further confirmed by transmission electron microscopic studies revealing a size domain ranging from 35-60 nm. Structural optimization of the pure host via first-principle routing disclosed the density of states and electronic band structure having a band gap of 4.738 eV, which was found to be quite comparable to the experimentally observed value (4.37 eV) from the diffuse reflectance spectra. Many key optical properties, such as optical conductivity, extinction coefficient, loss function, and refractive index were determined by using a calculated dielectric function. Furthermore, photoluminescence studies inferred that 10 mol% of Sm3+ ion in Ca9Al(PO4)7 host matrix gave the best luminescence emission intensity. Detailed photoluminescent analysis of Ca9Al(PO4)7:xSm3+ phosphors demonstrated that on excitation at 401 nm, an intense reddish-orange emission was received at 600 nm (4G5/2 → 6H7/2 transition). The critical distance for energy migration came out to be 22 Å which strongly favors the mechanism behind luminescence quenching as dipole-quadrupole interactions resulting from the over-doping of activator ions. The chromaticity coordinates (0.5216, 0.3630) were obtained using MATLAB computations. These results favor the applicability of Sm3+ doped Ca9Al(PO4)7 phosphors as the reddish-orange emitter in near-ultraviolet excited white light-emitting diodes.