Ab Initio Calculation of Charge-Transfer Absorption in CaF$_{2}$ and SrF$_{2}$ Crystals With Eu $^{3+}$ and Yb $^{3+}$ Impurities

In this paper we present the results of ab initio calculations of charge-transfer absorption bands in Eu
³⁺
and Yb
³⁺
-doped CaF
₂
and SrF
₂
crystals, which are due to transitions from the top of valence band to 4f state of trivalent rare-earth ion. The density-functional theory calculations have been performed in embedded-cluster approach. It was shown that low-energy absorption band is caused by charge-transfer transitions between 2p-states of interstitial fluorine and 4f state of the impurity ion.