Condensed-Matter Physics (1986) / Chapter Skim
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6 Defects and Diffusion
6 Defects and Diffusion
Pages 127-143
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From page 127... ...
Of particular interest are point defects, which are fairly localized on an atomic scale; line defects, such as dislocations; and boundary defects, such as surfaces, stacking faults, and grain boundaries. Although it is one of the older fields in condensed-matter physics and materials sciences, it remains an attractive arena for the observation and description of new physical phenomena and therefore maintains an enduring interest of physicists, in addition to researchers with other scientific orientations.
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From page 128... ...
The melting of adsorbed overlayers, or equivalently of impurity planes intercalated in layered compounds, to form a hexatic "floating raft" phase, is described in current theory by the thermally activated dissociation of dislocation dipole pairs. Similarly, the theoretical building blocks of the roughening transitions of surfaces and interfaces are just the steps and jogs of classical model surfaces.
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From page 129... ...
One is that the excess point defects produced and maintained by the radiation field may favor a state or solid phase different from the one that is stable in the absence of radiation. Examples of this are the formation of amorphous silicon, of highly supersaturated crystalline solid solutions, and of the disordered state of ordered alloys, all during irradiation.
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From page 130... ...
In chemical sensitivity SIMS can often achieve a level of 1 part in 106 or better. By Auger methods the sensitivity is reduced to 1 part in 103, but the depth resolution may be improved to a few atomic layers since the Auger electrons from deeper layers are scattered and lost.
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From page 131... ...
Alternatively, channeling methods can be used to probe the position of an atomic species in the lattice structure. When the beam is directed along a crystallographic axis so that the particle range is long, atoms located off crystal lattice sites scatter particles into other channels and into the bulk material.
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From page 132... ...
If not highly precise, these methods can nevertheless often reproduce systematic trends in data such as F-center excitation energies and Schottky pair energies with absolute values within 20 percent of those observed. Long experience, the systematic elimination of errors, and fine tuning of the codes have made the procedures reliable for ionic materials such as alkali halide and alkaline earth fluorides, which have large excitation energies and hence stable polarizabilities.
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From page 133... ...
Variational and Green's function Monte Carlo approaches to the specific problem of the electron liquid appear to offer feasible future routes. The difficulty in representing excited configurations, particularly those containing inhomogeneities such as charge localization, probably places serious limitations on the applications of the density functional method to areas of excited-state spectroscopy.
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From page 134... ...
This is particularly the case for excited configurations that differ in symmetry from the ground state, so that the two remain unconnected by pair excitations in the many-body perturbation theory. FUNDAMENTALS OF ATOMIC MOBILITY Until recently, computer simulation using molecular dynamics has provided the sole method by which jump rates can be calculated quantitatively for a given defect in a model crystal in which atoms interact through a specified potential function.
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From page 135... ...
The influence of quantum constraints on atomic jump processes has been the source of a large theoretical literature that has dwarfed the incidence of verifiable experimental reports citing observations of quantum effects. Well-established observations exist for tunneling of even quite heavy atoms among pocket states of asymmetric defect configurations; these results include, for example, off-center Cu substituted in salts and, more recently, Zn-A1 mixed dumbbell interstitials in Al.
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From page 136... ...
It is possible that exchange delocalizes vacancies in these materials and mixes them into the crystal ground state so that they are never absent in equilibrium, even at O K Explicit measurements have revealed that the vacancy content of hop 4He and bee 3He is below 1 part in 104 as T ~ 0.
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From page 137... ...
For both point and extended defects, therefore, the promise of remarkable defect behavior remains mostly to be verified in future work. COMMENTS ON ACTIVE AREAS What follows are brief descriptions of additional areas that seem particularly noteworthy in the light of past developments or potential future interest.
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From page 138... ...
These advance far beyond the resistivity measurements and occasional specific heat and Bragg x-ray scattering experiments available before 1970, and the prospects for steady future progress are improved accordingly. Surface Diffusion Only at temperatures typically below half the melting temperature does the diffusion of atoms on clean surfaces usually resemble the site-to-site hopping of atoms in the bulk crystal.
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From page 139... ...
A special opportunity therefore exists in the future to examine photochemical processes, including photon-induced point-defect migration, on the time scale of the atomic jump process itself. Molecular Dynamics Computer simulation of dynamical processes in solids has led to vivid insight into complex mechanisms, to the discovery of qualitatively new processes, and to quantitative mimicking of processes that
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From page 140... ...
More recent successes that warrant mention here include the accurate treatment of ionic motion in fast ion conductors such as AgI, in which the Ag sublattice disorders, and in CaF', where the F sublattice undergoes large fluctuations involving defects and mobility. Model interatomic forces have reproduced observed diffusion rates quantitatively.
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From page 141... ...
Vivid patterns of atomic distributions in the perfect crystal can be obtained for appropriate thin films. Dislocation and defect structures at properly aligned grain boundaries can also be imaged.
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From page 142... ...
SOME DIRECTIONS FOR FUTURE RESEARCH A qualitative understanding of phase microstructure and phase generation in radiation fields is developing, but detailed model descriptions and even the basic theoretical framework remain largely to be developed. One phenomenon, radiation-induced homogeneous precipitation in undersaturated solid solutions, has been described using a simplified quasi-thermodynamic theory.
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From page 143... ...
It is fairly clear that the discrepancies between the results of statistical theories and those of molecular-dynamics calculations for atomic jump rates in systems in which diffusion is occurring will be resolved over the next 5 years for model crystals with reasonably simple potential functions. Detailed properties of atomic jumps in model crystals, including isotope effects and thermodynamic derivatives of jump rates, will thus become accessible for the first time.
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