MINIMIZATION OF COHESION ENERGY FOR PARTIALLY COVALENT CRYSTALS USING SEMIEMPIRICAL INTERACTION POTENTIALS
Vadim S.Urusov
Department of Crystallography and Crystal Chemistry, Moscow
University, 119899 Moscow, Russia.
urusov@geol.msu.ru
Keywords: modelling structure and properties, partially
covalent crystals, charge transfer energy correction
It is well known that in many cases, to simulate structures and properties of partially covalent crystals, there is a need to involve variable atomic charges or ionicity parameters in the semiempirical interaction potentials to be optimized in the energy- minimization procedure. The potential function employed consists usually of an effective Coulomb and Van der Waals attractions, repulsion and covalent-type terms. However, no energy minimum exists when the calculations are performed for various values of the ionicity degree parameter of the crystal under study. The cohesion energy calculated for optimal sets of the structural parameters and some other properties can not be compared to any measurable quantity.
In such cases, to make the calculated energy comparable to the experimental estimate (atomization energy) there is a need to involve an additional intraatomic energy term, the so-called charge- transfer energy [1,2]. For this purpose a set of charge-transfer energies for valence states of many chemical elements is computed. A procedure based on the principle of the orbital electronegativities equalization is proposed to estimate the total contribution of the charge-transfer energy to the cohesive energy of a crystal. This technique allows to find a theoretical estimate of atomization energy and corresponding structural and other physical properties of a partially covalent crystal.
The calculations of structure, energy and elastic properties
were performed using the METAPOCS code with ionicity variation
for some halides (NaCl, CaF2), oxides (MgO, Al2O3,Cr2O3,
SiO2 quartz and stishovite,TiO2 and SnO2),
sulphide ZnS and silicates Al2SiO5 and
CaSnSiO5. The resulting cohesive energies usually
agree with experiments better than those calculated in purely
ionic or covalent approximations. The predicted elastic
properties of crystals under consideration are functions of the
ionicity parameter too and their values corresponding to the
optimized ionicity agree well with available experimental data.