Ab Initio Calculations of the Electronic Structure of Red-Emitting Mn4+-Doped Fluorides

Ab initio wavefunction-based quantum chemical calculations on the geometrical and electronic structure of 3d3 ions as dopants in different fluoride lattices are reported in this work. Mn4+-doped K2SiF6, a very promising candidate as red-emitting phosphor to be used in white light-emitting diodes, is studied in the first place. Using previous experience in similar calculations, we performed restricted active space self-consistent field calculations in which the multireference includes three active electrons in mainly Mn 3d, 4s orbitals plus singles and doubles excitations from the F 2p and Mn 3p orbitals, followed by RASPT2 calculations explicitly correlating 59 electrons. The structural parameters (Mn–F equilibrium distances and a1g vibrational frequencies) of the ground 4A2g (t2g3) multiplet, the intraconfigurational a2Eg, a2T1g, and a2T2g multiplets, and the interconfigurational 4T2g, a4T1g (t2g2eg), and b4T1g (t2geg2) multiplets are calculated using molecular orbitals individually optimized for the different states. Then, electronic transition energies between these states are computed. A large mixing between the levels of the a2T2g and 4T2g multiplets, due to spin–orbit coupling, is predicted and its consequences are analyzed. We find a systematic (good) agreement between calculated transition energies and experimental results, the spin-forbidden transitions deviate by only around 200 cm–1 (including the red emission a2Eg → 4A2g) and spin-allowed transitions are overestimated by 1000–1500 cm–1, partly as a consequence of the calculated equilibrium distances being too short. Because of this, the 10Dq parameter is somewhat overestimated. The same procedures are applied to Cs2GeF6:Mn4+, KMgF3:Cr3+, and KMgF3:V2+, and we find a very similar, reasonable agreement with experimental findings. This agreement makes us confident that the proposed recipes provide fairly accurate information at a reasonable computational effort for transition metals as dopants in fluoride crystals, both for low and intermediate oxidation states of the impurity.