J. S. Nadeau

Color Centers and the Flow Stress of LiF Single Crystals

Lithium fluoride crystals were exposed to cobalt‐60 gamma radiation at 300° and 78°K. Changes of the flow stress and of the optical absorption were measured in an effort to relate the hardening to identifiable color centers.In crystals irradiated at 300°K there was found to be a good correlation between the flow stress and the concentration of F centers. The magnitude of the hardening, however, suggested that it was caused by interstitial defects. Comparison of the hardening produced by additive coloring with that produced by irradiation indicated that the hardening defects were interstitial fluorines. This direct relation between interstitials and F centers supports a Varley type of mechanism for F‐center production in the bulk crystal. Some limited experiments with NaCl and KCl showed qualitatively the same behavior and suggest that the same F‐center production mechanism may occur in them.Irradiation of LiF at 78°K produced stable trapped‐hole centers and pulse annealing experiments showed that the Vk c...

Hardening of Potassium Chloride by Color Centers

The flow stress of KCl crystals containing color centers was studied in a systematic way to determine which color centers produced significant hardening. The color centers were introduced both by additive coloring and by gamma irradiation. Annealing and optical bleaching techniques were used to alter the relative concentrations of defects.For a given concentration of electron excess centers (F, M, R, N, etc.), the irradiated crystals were about ten times harder than the additively colored ones. Possible models for the halogen defect responsible for this high hardness were considered and it was concluded that it must be an interstitial ion or atom.Annealing of additively colored crystals indicated that the ``colloid band was actually two overlapping bands centered at about 735 and 760 mμ. Prior to formation of these bands the crystals became about 40% harder than in the as‐colored state indicating the presence of a coherent cluster of elementary defects. The flow stress decreased sharply upon appearance of...

Radiation Hardening in Alkali‐Halide Crystals

Hardening by gamma irradiation was investigated in eleven alkali halides with the rocksalt structure. The linear dependence of the flow stress upon the one‐half power of the F‐center concentration, as previously observed in LiF and KCl, was also observed in these new substances. (The earlier experiments showed that the hardening is not actually caused by F cneters but by a defect resulting from F‐center production.) The rate of hardening with concentration was found in all cases to be consistent with that to be expected for non‐symmetrical defects. Comparisons were also made between the amount of hardening per defect and the lattice spaces available to certain specific defects. The only clear correlation was with a halogen atom in the interstitial space at coordinates 0, ¼, ¼ in the unit cell.

Two Photomechanical Effects in Alkali Halide Crystals

Two photomechanical effects, observed in alkali halide crystals containing color centers, appear as sudden reversible changes in flow stress when a crystal is illuminated. Both effects are related to optical transitions of the F center. In potassium chloride, intensely illuminated at 175°K, hardening appears to be caused by the F → F′ process. Below 80°K, in KCl, even more severe hardening is caused upon illumination, by transport of the F‐center electron to an unspecified trap lying very close to the conduction band; decay of the hardening after extinction of the exciting light requires several seconds and may be caused by dislocation—point defect interactions.The effects have been observed in nine of the alkali halides. Flow stress increases and decreases were both observed, depending upon the particular alkali halide.

Hardening of Lithium Fluoride Crystals by Irradiation

The very soft, pure crystals of LiF that are now commercially available are unsuitable for some experimental work because they suffer accidental plastic deformation during cleavage and tend to split on (110) planes. Irradiation with gamma rays produces harder crystals that can be cleaved and handled without much damage. The radiation‐induced hardening and the color centers produced by moderate irradiation can be completely removed by annealing for several hours at 450°C. This method for handling very soft crystals may be useful for other types of crystals, and various types of irradiation can be employed.

HARDENING OF SILVER-DOPED POTASSIUM CHLORIDE BY LOW-TEMPERATURE X-RAY IRRADIATION

Silver‐doped KCl was much more susceptible than pure KCl to radiation hardening at low temperature. Pulse‐annealing experiments showed that a large fraction of the hardening was caused by self‐trapped holes, Cl2−. The silver ion which substitutes for the potassium ion in the structure performs the function of electron trap, thus stabilizing large numbers of isolated holes. The silver is relatively ineffective as a hardening agent in any of its several ionization states. This indicates that electrical interactions between dislocations and point defects are not an important source of hardening in ionic crystals. Just as with metals, the strongest hardening effects are caused by elastic interactions.

Temperature Gradient Induced Migration of Gas‐Filled Bubbles in KCl

The temperature gradient induced migration of gas bubbles has been studied in KCl. This material was selected because of its high vapor pressure and transparency. The experimental variables of temperature, temperature gradient, and diffusion coefficient of the vapor species were investigated and the observed velocities were shown to agree with those expected from the theoretically derived model. The ratio of the temperature difference across a bubble of low thermal conductivity to that across an equal length of the surrounding solid was determined from analogue experiments.

Morphology of Migrating Bubbles in KCl Crystals

Gas bubbles migrating under the influence of a temperature gradient in KCl gradually evolve stable shapes which have a low length (in the direction of migration) to width ratio and are bounded by cube planes (100). The shape is determined by evaporation‐condensation kinetics and not by the equilibrium surface energies. A significant part of the bubble volume is left behind in a trail of very fine bubbles resulting in as much as a fivefold increase in bubble surface area. Most of the observed phenomena can be correlated with the detailed temperature distribution around a migrating bubble.

Deformation of Dislocation‐Free Platelets of Nickel Bromide

Nearly perfect platelets of nickel bromide, a compound which normally has the CdCl2 structure, were deformed on a straining stage inside the electron microscope and observed in transmission. Widely extended ribbon dislocations were produced at the platelet edges and moved entirely on the layer planes. Interactions between sets of ribbon dislocations with Burgers vectors at 120° resulted in the formation of networks which, because of the low stacking fault energy in this material, consisted of a triangular mosaic pattern of stacking faults extending over large areas of the crystal. Two types of stacking fault, having energies in the ratio ∼1:2, were enclosed by the ribbon dislocations. The fault with the lower energy (γ/μ∼5×10−12 cm), which occurred most frequently, corresponds to a layer of CdI2 stacking and accounts for the presence in the x‐ray powder pattern of lines characteristic of the CdI2 structure. It results from translation between adjacent bromine sheets. The higher energy fault (γ/μ∼10−11cm)p...