High-resolution electron microscopy of dislocations of MgO
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K. Suzuki Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan
T. Suzuki Institute of Industrial Science, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan (Received 9 May 1994; accepted 28 July 1994)
Dislocations in MgO introduced by ion irradiation and by plastic deformation are observed by HREM. Depending on the Burgers vector and the dislocation character, various types of lattice images are obtained. Image simulations are performed for the inclination of dislocations, as well as for dissociated dislocations. A comparison of observed and simulated images shows that inclination of nondissociated dislocations makes them appear as if they were dissociated; in reality a/2(110) dislocations in MgO are not dissociated.
I. INTRODUCTION The core structure of dislocations is closely related to crystal plasticity—the choice of slip plane and the magnitude of the Peierls stress. Dislocations in ionic crystals with the NaCl structure have the Burgers vector b = a/2(110). The primary slip plane is usually {110} and not {001}, which has the widest spacing. Fontaine1 and Fontaine and Haasen2 pointed out the possibility of dissociation on {110} planes from calculations of stacking fault energies. Tasker and Bullough3 calculated the stacking fault energy, considering lattice relaxation, and obtained values 2 - 3 times as high as Fontaine. The core structures of dislocations in the NaCl lattice were simulated atomistically.4"10 Puls and So11 investigated the dissociation in NaCl by careful computation, using the flexible boundary method and taking into account the polarization of ions. They conclude that dislocations should not be dissociated. High-resolution electron microscopy (HREM) has been applied to various kinds of crystals for investigation of atomic configurations around dislocation cores, but the application to ionic crystals is limited.12 Takeuchi et al.13 have examined NiO, CoO, and MgO and reported that some dislocations are not dissociated, while some in NiO and MgO seem to be dissociated as a/2[110](lT0) = a/2[100] (0T0) + a/2[010] (100). Foitzik et al.14 examined PbS, which has the NaCl structure, and showed dissociation on both {001} and {110} planes. PbS is highly covalent and the primary slip plane is {001}, unlike in the ionic crystals with NaCl structure. Alkali-halides are heavily damaged during TEM observation, while oxide crystals have higher resistance to electron irradiation. MgO is a typical ionic crystal with J. Mater. Res., Vol. 9, No. 11, Nov 1994 http://journals.cambridge.org
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the NaCl structure, its plastic properties (the slip systems and the Peierls stress) are essentially the same as those in alkali-halides,15 and we need not consider nonstoichiometry effects that can affect the properties of dislocations in CoO 16 and NiO. 17 For these reasons we choose MgO for HREM, although its small lattice constant makes the contrast of lattice images relatively low. In MgO the dislocation density created by plastic deformation i
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