Structural and spectroscopic investigation of a novel orange-red Ca9Bi(PO4)7:Sm3+ nano-scaled phosphor

Abstract

A series of Sm3+ activated Ca9Bi(PO4)7 novel tunable orange-red-emitting nanophosphors with an intense emission band at 600 nm were synthesized using the solution combustion method. The crystal structure, phase purity, structural parameters, optical band gap values, photoluminescence (PL) behavior, energy transfer, quantum efficiencies, lifetime values, and CIE chromaticity index of Ca9Bi(1-x)Smx(PO4)7 white powdered samples were consistently analyzed. Rietveld refinement of Ca9Bi0.88(PO4)7:012Sm3+ sample resulted into a trigonal crystal structure of sample having space group R3c(161) with refinement converged to Rp = 7.62%, Rwp = 12.01%, and ꭓ2 = 2.08. Meanwhile, the average crystallite size for Ca9Bi0.88Sm0.12(PO4)7 system was found out to be nearly 60 nm which was also supported by TEM images. The energy band-gap (Eg) value of Ca9Bi0.88Sm0.12(PO4)7 was estimated to be 4.07 eV and the critical distance between Sm3+ ions was calculated to be 21.09 Å. The optimum concentration of dopant Sm3+ ions in Ca9Bi(PO4)7 sample was ascertained to be 12 mol% and as per Dexter's theory and Huang study, the mechanism behind energy transfer between Sm3+ ions was concluded as dipole-dipole interactions. Under excitation at 403 nm, these nanophosphors exhibited three bands in the PL emission spectrum peaking at 562 nm, 600 nm, and 646 nm corresponding to 4G5/2 → 6HJ transitions (J = 5/2, 7/2, and 9/2, respectively). A decrease in lifetime values from 1.95 ms to 1.39 ms was observed with increased Sm3+ ions concentration. Upon employing Auzel's fitting function, radiative lifetime; non-radiative rates, and quantum efficiencies for as-synthesized nanophosphors were measured. The CIE color coordinate of Ca9Bi0.88(PO4)7:0.12Sm3+ was found to be (0.435, 0.354) and the corresponding quantum efficiency was estimated at 70.6%. The results obtained from our current study indicate that Ca9Bi(PO4)7 doped with Sm3+ ions can have applicability as a promising orange-red-emitting nanophosphor for near-UV energized WLEDs.