Reddish-orange emission from sol-gel derived Sm³⁺-doped Sr₂La₈(SiO₄)₆O₂ phosphors

In this study, Sr₂La_8-x(SiO₄)₆O₂:xSm³⁺ (where x=(0.01 ≤ x ≤ 0.11)) phosphors were prepared successfully via the sol-gel route. Their structural properties were evaluated through fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) measurements. The XRD patterns confirmed the purity of the prepared compound that had an average crystallite size of ∼38.8 nm. From the FTIR spectra, various stretching and bending vibrations of the SiO₄ units were identified. The band observed at 540 cm⁻¹ was attributed to the normal vibration mode of La-O. The optical properties of the prepared phosphor were assessed through diffuse reflection spectroscopy and photoluminescence measurements. The diffuse reflectance spectrum exhibited seven peaks from the Sm³⁺ ions corresponding to f-f transitions from the ground state, ⁶H_5/2, to the excited states, ⁶G_7/2, ⁴D_7/2, ⁴D_3/2, ⁶P_7/2, ⁴L_15/2, ⁶P_3/2, and (⁴I_11/2,⁴I_13/2). Out of all these transitions, the peak at 407 nm is the most prominent, this peak come from the ⁶H_5/2→⁶P_3/2 transition, which offers the maximum absorption. The phosphor's excitation spectrum revealed a charge-transfer (CT) band that extended from 220 to 270 nm and was centered around 240 nm. The excitation spectra also exhibited several transitions of the Sm³⁺ ion, among these, the transition corresponding to 403 nm was chosen as the pumping wavelength owing to its prominence. The most intense emission peak was observed at 599 nm, this corresponds to the ⁴G_5/2→⁶H_7/2 transition. This emission's CIE chromaticity coordinates placed it in the reddish-orange region, suggesting that the titled phosphor is suited for orange-red LEDs and the red component in WLEDs.