Experiments and theories concerning charged dislocations in alkali halide crystals are reviewed in detail, with particular attention to the way in which the experiments should be interpreted and to the range of applicability of the sweep-up and various forms of thermal-equilibrium models. Possible effects on mechanical properties and internal friction are analysed. The transient and steady-state effects of plastic deformation on ionic conductivity are described and new interpretations involving charged dislocations are proposed. A survey of results on the scattering of light by alkali halide single crystals leads to the conclusion that charged dislocations do not play an important role. Evidence about charges on surfaces and on dislocations in AgCl and AgBr is reviewed and compared with that for alkali halides. Comparisons are also made with MgO, the CsCl and CaF2 structures, semiconductors and ice.

Charged dislocations in ionic crystals

Special aspects of diffusion in thin films

This review considers the calculation of point defect parameters in ionic solids by atomistic simulation. Both static and dynamic methods are discussed. After a brief overview of the field, the author considers how to obtain a reliable model of the crystal potential. This matter is fundamental to any adequate simulation. The author then turns to consider the calculation of defect energies and entropies. The following section discusses how the results of such simulations are to be interpreted. This requires a discussion of the reaction rate theory for defect motion, in particular recent work on what the theory means and how its shortcomings can be corrected. The author then turns to the interaction of point defects with interfaces and to their interaction with dislocations. The effects of the long-range electrical terms are shown to be particularly important in these circumstances. Finally, the author summarizes the review and discuss future directions in the field.

https://iopscience.iop.org/article/10.1088/0034-4885/53/11/002/pdf

Computer simulation of defects in ionic solids

The segregation of calcium to a (100) cleavage surface of an MgO crystal, with bulk calcium concentration of 200 ppm was measured in situ at T=900° to 1450°C in ultrahigh vacuum, using Auger and low-energy ion-scattering spectroscopies. A measured heat of segregation of approximately -50.3 kJ/mol (-12 kcal/mol) is in favorable agreement with a value of -58.7 kJ/mol (-14 kcal/mol) determined using solute strain energy and surface free energy criteria. The equilibrium value for the calcium segregation between 950° and 1000°C is estimated to correspond to a 20% occupation of the surface cation sites.

Calcium Segregation to a Magnesium Oxide (100) Surface

A quantitative study of space charge solute segregation at grain boundaries in TiO[sub 2] is conducted, using a new STEM method for the measurement of aliovalent solute accumulation. It is shown that the electrostatic potential at grain boundaries can be varied in sign and magnitude with doping, oxygen pressure, and temperature, and that the isoelectric point lies in slightly donor-doped compositions for samples annealed in air. The experimental results closely fit the space charge model in Part 1. Space charge solute segregation is found even in defect regimes of high electron concentration. Approximately one in ten grain boundaries are special in exhibiting no detectable segregation; in one such instance a twin boundary is identified. Among boundaries with significant amounts of segregation, clear differences in potential also exist. From the potential determined in acceptor- and donor-doped composition, the Frenkel energy (assumed to be lower than the Schottky energy in TiO[sub 2]) can be separated into its individual terms. An average value for the titanium vacancy formation energy of g[sub v[sub Ti]] = 2.4 eV and an upper limit to the titanium interstitial formation energy of g[sub Ti[sub i]] = 2.6 eV are obtained.

S. Danyluk

Papers

Space charge regions at silver halide surfaces: Effects of divalent impurities and halogen pressure

Space charge regions at silver halide surfaces: Experimental results for undoped AgCl