The average values for each family are 10.7, 10.3, 9.7, and 9.4 eV, respectively, for MgF 2 , CaF 2 , SrF 2 , and BaF 2 and exhibit a decreasing trend with increasing size of the metal atom; for a given metal atom, they decrease when going from n = 1 to n = 3 with a spread of about 1.0–0.75 eV.
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