Energy minimum criteria in modeling structures and properties of minerals |
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| Authors: | V S Urusov N N Eremin |
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| Institution: | 1. Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Kosygin street, 19, 117975, Moscow, Russia
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| Abstract: | The leading principle in modeling procedures is the minimization of structural energy. It is assumed that the calculated minimal energy has to be compared with the experimental estimate of cohesion energy of a crystal. The reference state for structure energy depends on bonding type: the lattice energy for purely ionic crystals consisting of cations and anions, the atomization energy for covalent and metallic crystals consisting of atoms, the sublimation energy for molecular crystals consisting of molecules etc.As is well-known, a majority of minerals can not be correctly described as purely ionic crystals. Moreover, for these crystals lattice energy cannot be determined empirically because free anions, such as O2-, S2-, As3-, etc., do not exist. In order to describe crystal structures and properties in a better approximation it is usually proposed that the bonding character of such crystals is intermediate between ionic and covalent, so that effective charges instead of formal ones, and an appropriate covalent contribution, are involved in energy calculations. However, the corresponding calculated structure energies are not comparable to any experimental values. Moreover, they decrease with increasing effective charges and a purely ionic structure seems, as before, to be most stable from energetic point of view. |
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