First-principles and crystal-field calculations of the electronic and optical properties of two novel red phosphors Rb₂HfF₆:Mn⁴⁺ and Cs₂HfF₆:Mn⁴⁺

The electronic, structural, and optical properties of 2 red phosphors, Rb₂HfF₆:Mn⁴⁺ and Cs₂HfF₆:Mn⁴⁺, are evaluated using the first-principles and crystal field theory methods. The calculated trigonal splitting of the Mn⁴⁺ orbital triplets perfectly matches the experimental excitation spectra. The structural and electronic properties of the mixed compound RbCsHfF₆ are also studied theoretically. In the mixed compound, the inversion center symmetry around the Hf site is removed. This symmetry lowering may result in an increase in the Mn^{4+ 2}E→⁴A₂ zero phonon line (ZPL) intensity, which is very weak in the 2 end members. This finding may be of interest for increasing the phosphor luminosity. It is believed that such a mechanism of local site symmetry lowering by preparing solid solutions may be used for other systems as well, to gain ZPL intensity and perhaps to minimize thermal losses, eventually leading to improved phosphor materials.