Functions of Isotopic Forms (
-Factors) for Ionic Crystals
A. V. Bochkarev*, A. N. Trefilova*, and M. F. Bobrov**
* Timiryazev Agricultural Academy, Russian Academy of Sciences, ul. Timiryazeva 47, Moscow, 127550 Russia
** Mendeleev University of Chemical Technology, Miusskaya pl. 9, Moscow, 125190 Russia
Received June 4, 2001
Abstract—The cluster approach to calculations of the ratio between the partition functions of isotopic forms
(
-factors), which was earlier applied to molecules and covalent and metallic crystals, was extended to ionic
crystals. It was shown that, as distinguished from molecular crystals and crystals of the other types, crystal lat-
tice fragments cut from real ionic crystals could not be used in calculations as a satisfactory approximation
because of the long-range nature of Coulomb forces. Correct calculation data on -factors coinciding with those
obtained by the other methods can be attained if ionic clusters are calculated taking into account their real equi-
librium geometry. Various approaches to modeling short-range interatomic interactions in ionic crystals were
studied, including ab initio quantum-chemical methods and the method of empirical interatomic pair potentials.
It was shown that, among the empirical interatomic potentials, the Born–Mayer exponential potential was the
best one for calculating -factors. The -factors were calculated for several ionic crystals with NaCl-type lat-
tices.
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