Maxwell-Boltzmann Distribution of M 3+-F− Interstitial Pairs in Fluorite-Type Lattices

Maxwell-Boltzmann distribution curves for M3+-F− interstitials in fluorite-type lattices have been calculated and compared with spectroscopic data. The dominant sites, in the order of their relative importance for low concentrations and T<700 K, are the C4v(1, 0, 0), Cs(2, 1, 0), C3v(1, 1, 1), Cs(1, 2, 2), Cs(1, 1, 3), and Cs(2, 3, 0) sites. The dominant presence of the third-nearest-neighbor monoclinic Cs(2, 1, 0) pair, which has not been reported previously, is predicted by our calculations. In view of this result, the electron spin resonance data on U3+ in CaF2 reported by Mahlab, Volterra, Low, and Yariv, which these authors interpreted in terms of the fifth-nearest-neighbor site, have been shown to arise actually from the prominent presence of the third-nearest-neighbor Cs(2, 1, 0) site.