Calculation of energy of pairwise interatomic interaction with use of the ab initio bachelet-hamann-shluter electron-ionic pseudopotentials

摘要

A general treatise of effective interatomic potentialin covalent crystals is developed. It is suggested that an interatomicinteraction consists of two components. The first (direct)component is the Coulomb repulsion, and the second componentis the (indirect) attraction via the free electrons. Indirectcomponent is founded in the second-order theory of perturbationson pseudopotential. The results of numerical calculations ofinteratomic potential for atoms of Si, C, and O inYBa_2Cu_3O_x are presented. Connection between oxygenordering and superconductive transition in ceramicYBa_2Cu_3O_x is examined. Formula for free energy of thissystem is deducted using interatomic potential. The values ofinteratomic potential at the distances according to positions ofoxygen vacancies are calculated. Taking into account these results,a dependence of free energy of this system is analysed, and relevantgraph is plotted. Obtained data allow us to conclude thatsuperconductive ceramic YBa_2Cu_3O_x is maximally ordered.It is calculated the dependence of ordering energy of Si and Catoms on the distance. Results of analysis of this dependence forgiven structures of carborundum polytypes are in a goodagreement with experiment. According to computation of thecouple interatomic potential, the averaged values of electrostaticenergy per atom for 3C and 2H polytypes are calculated. Theydiffer by value of about 0.2 e V that corresponds to the SiC statediagram. The value of obtained difference let us to suggest that thepolytypic transformation can be realized through liquid or vapour phase.