Computer Modelling of Energy Structure of Yb3+ and Lu3+ Doped LaF3 Crystals

The energy band structure, as well as partial and total densities of states have been calculated for LaF3:Yb and LaF3:Lu crystals within density functional theory using the projector augmented wave method and Hubbard corrections (DFT + U). The influence of geometric optimization on the results of energy band calculations of LaF3:Ln crystals (Ln = Yb, Lu) was analysed and the absence of relaxation procedure is confirmed to negatively influence the energy position of states, and the variability between obtained results of different optimization algorithms are within the calculation accuracy. The top of the valence band of LaF3 is confirmed to be formed by the 2pF--states and the bottom of the conduction band is formed by the 5d-states of La3+. The positions of the 4f-states and 5d-states of activator ions in LaF3 were studied. It is shown that the 4f-states of Yb3+ are slightly above the top of the valence band and the 4f-states of Lu3+ to be 3.5 eV below the top of the valence band. The energy levels of the 5d states of the impurities are energetically close to the bottom of the LaF3 conduction band. The calculated band gap of 9.6 eV for LaF3 is in a good agreement with the experimental result and is not affected by impurity ions

Energy Structure and Luminescence of CeF3 Crystals

The results of the calculation of the energy band structure and luminescent research of CeF3 crystals are presented. The existence of two 5d1 and 5d2 subbands of the conduction band genetically derived from 5d states of Ce3+ ions with different effective electron masses of 4.9 me and 0.9 me, respectively, is revealed. The large electron effective mass in the 5d1 subband facilitates the localization of electronic excitations forming the 4f-5d cerium Frenkel self-trapped excitons responsible for the CeF3 luminescence. The structure of the excitation spectra of the exciton luminescence peaked at 290 nm, and the defect luminescence at 340 nm confirms the aforementioned calculated features of the conduction band of CeF3 crystals. The peculiarities of the excitation spectra of the luminescence of CaF2:Ce crystals dependent on the cerium concentration are considered with respect to the phase formation possibility of CeF3