Computer Modelling of Energy Structure of Yb 3+ and Lu 3+ Doped LaF 3 Crystals.

The energy band structure, as well as partial and total densities of states have been calculated for LaF 3 :Yb and LaF 3 :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 LaF 3 :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 LaF 3 is confirmed to be formed by the 2pF - -states and the bottom of the conduction band is formed by the 5d-states of La 3+ . The positions of the 4f-states and 5d-states of activator ions in LaF 3 were studied. It is shown that the 4f-states of Yb 3+ are slightly above the top of the valence band and the 4f-states of Lu 3+ 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 LaF 3 conduction band. The calculated band gap of 9.6 eV for LaF 3 is in a good agreement with the experimental result and is not affected by impurity ions.