Rationalizing the structural changes and spectra of manganese and their temperature dependence in a series of garnets with first-principles calculations

Detailed first-principles calculations have been carried out to study the stabilization, excitation and luminescence mechanisms of the ion Mn3+ in a series of A3B2B3′O12 garnet hosts. The formation energy shows that Mn3+ is dominant and is situated at the octahedral B site. The excited states, excitation, and emission energies of Mn3+ have then been calculated. The calculated energy levels of Mn3+ confirm that the red emission is due to the 5T2→5E′ transition and the near-infrared (NIR) emission arises from the 1T2→3T1 transition. The populations of the 5T2 and 1T2 excited states and the corresponding radiative rates lead to the temperature dependence of the red to NIR emission. Furthermore, the adiabatic potential energy surfaces along the A1g and Eg moeity modes of [MnO6] have been calculated and fitted well in the harmonic approximation. The high activation energy for Mn3+ indicates a low nonradiative multiphonon relaxation rate of 5T2 to 3T1. Hence, the ionization process was considered, and we show that it is responsible for the luminescence quenching of Mn3+, so that the luminescence has rarely been reported experimentally. This work illustrates a well-designed approach based on the density-functional theory framework to predict the optical transition properties of the transition metal ion Mn3+ by calculating the structural distortions due to the Jahn-Teller effect, the optical transitions, quenching processes and the influence of pressure.