Dislocations In Ionic Crystals Research Articles

Influence of environment on the mobility of near-surface dislocations in ionic crystals

The environment-sensitive mobility of near-surface edge and screw dislocations around indentations in LiF, MgO, and NaCl has been studied. It is demonstrated that, contrary to recent reports, adsorbed species can markedly affect dislocation mobility in the absence of either surface step formation or significant reductions in surface free energy. The mechanistic significance of this is discussed.

High-temperature dislocation mobilities in doped LiF

Abstract The stress and temperature dependences of the dislocation velocity in LiF doped with 80 p.p.m. Mg have been indirectly measured in the temperature range 773° to 973°K (0.7 to 0.8 T m). These measurements are similar to those made in a previous study of dislocation mobilities in ’pure’ LiF in that they are based on a study of the sigmoidal creep behaviour of single crystals. This technique for obtaining dislocation mobilities at high temperatures, which was validated in our previous study, is based on Haasen's theory of sigmoidal creep. The stress dependence of the dislocation velocity as characterized by a stress exponent, m, is large, as in the case of ‘pure’ LiF, and varies significantly with temperature. The stress exponents range between 6 and 15 and reach a maximum near 893°K. The temperature dependence of the dislocation velocity is unusual and cannot be characterized by a single activation energy within the experimental range. The dislocation velocity depends very sensitively on temperature near 893°K. A qualitative interpretation of the unusual dislocation mobilities of both ‘pure’ and doped LiF, based on the effects of electric charges on dislocations in ionic crystals, is presented. The rate-limiting mechanism for dislocation motion is assumed to be the diffusional drag of the space-charge clouds of point defects which surround the dislocations in these crystals.

Dislocation Jog and Kink Interactions in Ionic Crystals

AbstractJogs and kinks on dislocations in ionic crystals interact both elastically and electrically. In cubic crystals the electrical interaction can be calculated by simple isotropic theory. Anisotropic calculations of the elastic interaction have been performed for a number of ionic crystals of the NaCl type in order to decide whether the net interaction is attractive or repulsive and whether neutral superjogs tend to form.

Dissociation des dislocations sur les plans (110) dans les cristaux ioniques du type NaCl

It is suggested that dislocations in ionic crystals of the NaCl type may be dissociated in (110) planes according to the reaction a 2 [110] → a 4 [110] + a 4 [110]. A rough calculation gives an estimation of the energy for the stacking fault between the two partials. One thus obtains dissociations of the order of some b. A model for the formation of sessile locks is proposed.

DISLOCATIONS IN IONIC CRYSTALS (Structure, Charge Effects and Interaction with Impurities)

DISLOCATIONS IN IONIC CRYSTALS (Structure, Charge Effects and Interaction with Impurities)

The Charge of Dislocations in Alkali Halide Crystals

AbstractA method of determining the electrical charge on dislocations in ionic crystals from vibration experiments on bent crystals is presented. For a typical case two approximately equal values (σ = 1.0 × 10−3 and 0.8 × 10−3 esu/cm) are obtained using different approximations.

Charged Dislocations in Ionic Crystals

The paper aims to show that the charge clouds around dislocations are important for the mechanical properties of ionic crystals. The previous work of Eshelby, Newey, Pratt, and Lidiard is extended by solving the nonlinear equation giving the potential distribution and by obtaining the boundary condition appropriate at the dislocation core. Three typical examples are worked out in detail. Finally, a series of experimental phenomena are discussed and it is shown how the charge cloud model can be used to understand the experimental behavior. In particular, the minimum in the yield stress versus annealing temperature of NaCl doped with a known concentration of Pb${\mathrm{Cl}}_{2}$ is used to obtain the energy of formation of a positive ion vacancy in NaCl. One finds that ${{E}_{F}}^{+}=0.53$ eV.

Sign of Charged Dislocations in NaCl

An interpretation of previous experiments on the movement of charged dislocations in ionic crystals is given. It is shown that those experiments involving bending do not allow an unambiguous determination of the sign of the charge to be made and the contradictory results obtained from indentation experiments can be resolved on the assumption that the dislocations carry a positive charge.

Vacancy Pairs and Dislocations in Ionic Crystals

(1) We have calculated the binding energy of vacancy pairs in NaCl and the other five alkali halide crystals. The calculations is improved at several points compared with that by Reitz and Gammel. The calculated values of the binding energy are considerably smaller than e2/ka and differ from the result by Reitz and Gammel; for instance, 0.44 ev in NaCl, 0.37 ev in NaBr, and 0.58 ev in KCl crystals. (2) As the first step of the systematic investigations of dislocation problems, we have calculated the core energy and the Peierls-Nabarro critical yield stress of the edge dislocation for slip in the (110) plane in NaCl type crystal [the Burgers vector is a (110)]. If we write the energy of the dislocation at the center of a cylinder with radius R as K ln (R/R0), R0 is about 1 A in most of the alkali halide crystals. The results of the calculation concerning the Peierls-Nabarro yield stress permit us to understand the well-known fact that LiF has much larger critical yield stress compared with the other alkali halide crystals.

Charged Dislocations in Ionic Crystals

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Observations on charged dislocations in ionic crystals

Abstract Further results concerning charged dislocations in NaCl and LiF are described. These observations differ from those published earlier owing to the fact that the duration of the deformation pulses could be changed. It was found that in most of the specimens the dislocations were negatively charged and a ‘barrier formation’ is described. A second series of experiments with an Instron tasting machine showed that an electrical effect could be obtained in the elastic region of the specimen.