Preparation and Luminescence Properties of Mn4+ Ions Doped CaAl12O19 Red Phosphors

The present paper details the preparation of CaAl 12- x O 19 : x Mn 4+ (CAO: Mn 4+ ) red phosphors based on CaAl 12 O 19 using a high-temperature solid-state method through Mn ion doping. A range of analytical techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM) and fluorescence spectroscopy, to comprehensively investigate the crystal structure, microstructure and photoluminescence characteristics of the samples. The results demonstrate that the doped Mn 4+ ions enter the octahedral sites of Al 3+ , and the lattice constants of the matrix does not change significantly because the ionic radius of the doped Mn 4+ ions is equivalent to the size of the Al 3+ ions. On this basis, the lattice size and lattice strain were calculated using the W-H equation. The results show that as the doping concentration gradually increases, both the lattice size and strain exhibit a slowly increasing trend. The prepared phosphor exhibited a polyhedral morphology, characterised by two hexagonal prisms with irregular hexagonal end faces, a substantial particle size, and optimal dispersion. When exposed to near ultraviolet light at a wavelength of 349 nm, the photoluminescence spectrum displays narrow-band emission characteristics, with the central wavelength of the emission peak at 654 nm and the full width at half maximum (FWHM) of 40.706 nm. The emission peak is attributed to the transition from 2 E g to 4 A 2g of Mn 4+ in a strong crystal field. With the increase of Mn 4+ doping concentration, the fluorescence intensity first increases and then decreases, and the fluorescence intensity is the highest when the doping concentration is 1.2 mol%. The analysis yielded three emission peaks, which were separated by the peak separation method. These peaks correspond to the emission of Mn 4+ at three different octahedral points of Al 3+ . The integrated area ratio of the three peaks indicates that Mn 4+ is predominantly present on the symmetrical sites. Thermal stability analysis demonstrates that the emission intensity of the phosphor decreases rapidly due to three factors. Firstly, the size of the Mn 4+ ions is smaller than the octahedral gap size. Secondly, the energy is higher. Thirdly, no radiation transition intensifies with the increase of temperature. The colour purity calculation indicates that the red phosphor prepared has a colour purity of over 99%. At the same time, the color temperature is below 6000k, which renders it suitable for indoor plant cultivation.