Photoluminescence and Raman Scattering Spectra in (NH₄)₂XF₆:Mn⁴⁺(X = Si, Ge, Sn, and Ti) Red Phosphors

(NH 4 ) 2 XF 6 :Mn 4+ (X = Si, Ge, Sn, and Ti) red phosphors are synthesized by chemical reaction in (NH 4 ) 2 XF 6 /HF/KMnO 4 /NH 4 F mixed solutions. X-ray diffraction curves indicate that the synthesized phosphors have a cubic symmetry with the space group (X = Si) and a trigonal symmetry with the space group (X = Ge, Sn, and Ti). Fourier-transform infrared spectroscopy, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and Raman scattering techniques are used to investigate the optical properties of the phosphors. The Franck − Condon analysis of the PLE spectra yields the Mn 4+ -related transitions to occur at ∼2.65 eV ( 4 A 2g → 4 T 2g ) and at ∼3.4 eV ( 4 A 2g → 4 T 1g ) in these phosphors. Temperature-dependent PL measurements are carried out from 20 to 350 K in steps of 10 K, and the obtained results are interpreted by taking into account the Bose−Einstein occupation factor. A complete set of internal vibronic frequencies, ν 1 − ν 6 , for the ions in (NH 4 ) 2 XF 6 :Mn 4+ are determined from the PL and Raman scattering measurements.