On The1/2<110>{110} Dislocation Properties In The Nacl Structure Ionic Crystals

Dislocations are the most abundant defects in materials that play a prominent rolein controlling the properties of materials, in particular their mechanical behavior. Thecrucial problem of the dislocation is the determination of the core structure. Theclassical Peierls-Nabarro （P-N） model can determine the core structure and Peierlsstress quantitatively. However, the major drawback of the P-N model is the adoption oflinear elastic theory and the discreteness effect of crystals is underestimated. Besides, inthe classical P-N model, the nonlinear interaction of the misfit planes is described byemploying the sinusoidal force law, obviously, it is inaccurate for the real materials.With the revealment of the relationship between the misfit energy and the generalizedstacking fault energy （-surface）, the classical P-N model has been improved greatly,however, the discreteness effect of crystals is still unsatisfactory and needs furtherinvestigations. Recently, the full discrete lattice theory of dislocation based on the latticestatics has been constructed and the unified dislocation equation to discuss the corestructure and the Peierls stress is provided that can recover the classical P-N model. Inthis paper, the core structure and Peierls stress of the1/2<110>{110} dislocations inNaCl structure alkali halide crystals by using the dislocation lattice theory （the modifiedP-N model） combining the-surface calculated from the first-principle calculations.The main work and results involve:（1）1/2<110>{110} dislocations in NaCl structure alkali halide1/2<110>{110} slip system is the most easiest slip system in NaCl structurecrystals（LiF, NaF, KF, LiCl, NaCl, KCl）. There have been a great deal of numericalsimulations based on an atomistic description of the dislocations focusing on thecalculation of the Peierls stress of the1/2<110>{110} dislocation in NaCl structurecrystals, but all the results are larger than experimental values （approximately one orderof magnitude）. For example, the numerical calculation of the Peierls stress of the1/2<110>{110} dislocation in NaCl are6¹8times larger than the experimental results.The main reason is the model potential employed by these numerical simulations hascertain deviation. Therefore, it is meaningful in theory to investigate the1/2<110>{110}dislocations in NaCl structure alkali halide by using the modified P-N model. Weemploy the first principle calculation to calculate the elastic constants and thegeneralized stacking fault energy in these crystals. Using the calculated elastic constants we obtained their elastic anisotropic factor A, we find our researched alkali halidecrystals are all anisotropic crystal. Therefore, it is necessary to consider the effect of theelastic anisotropy to the dislocation core structure. Using the Foreman method to solvethe modified P-N model in the elasticity anisotropic approximation, the obtained Peierlsstresses for the edge dislocation in NaCl and KCl are0.46×10^-3μand1.19×10^-3μ,respectively. Which are agree well with the experimental values.（2） Pressure influence on the1/2<110>{110} dislocations in MgO and CaOIn this paper, we further discuss the pressure effect on the influence of thedislocation properties, considering the phase pressure of the alkali halide differentiatewith each other, and they have smaller phase pressures （the phase pressure of KCl isonly2.2GPa, the phase pressure of NaCl is29.0GPa）, they are disadvantaged for theresearched and comparison of the pressure effect. So we mainly research the1/2<110>{110} dislocation in alkaline earth oxides MgO and CaO in this paper,because they have lager phase pressure. Using the modified P-N model combined withthe generalized stacking fault energy and elastic constants under different pressureobtained from the first principle, the core structure and Peierls stress are calculated forthe1/2<100>{110} dislocation in NaCl structure ion crystal oxides MgO and CaO. Ourcalculated Peierls stress for the1/2<100>{110} dislocation in MgO at0GPa are40MPa and160MPa, respectively, which agree well with the experiments. Besides, forthe same crystal, the unstable stacking fault energy and the Peierls stress increase withthe increasing pressure, and the core width decrease with the increasing pressure.Furthermore, the Peierls stress of the edge dislocation in MgO and CaO under differentpressures is smaller than that of screw dislocation and mixed dislocation, this alsodemonstrates the phenomenon obtained from the experiment the edge dislocation isconsidered to dominant factor in determining the plastic behavior of the NaCl structure.But CaO has the opposite trend at100GPa, this may be resulted from that CaO hasB1-B2transition while the pressure larger than60GPa.