Graduate Thesis Or Dissertation
 

Dipole relaxation effects in doped potassium chloride by the double crystal method

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  • The investigation of impurity effects on the properties of alkali halide crystals has often been difficult because of the variation of procedures used in various experiments. The purpose of this work is to develop a self consistent method of measuring the IV dipole polarization properties of single crystals while eliminating the effects of the electrodes and the host lattice in the temperature range of -50 to 0°C. This was done by applying Kirchhoff's rule to a circuit which included the sample being studied and a pure reference sample. By allowing the two crystals to charge and discharge in opposition, the effects due to IV dipoles can be separated from other relaxation processes in the samples. This is called the Double Crystal Method (DCM). Investigations of high purity, OH⁻ free, potassium chloride doped with lead and cadmium yielded activation energies (ε) of 0.691 ± .01 ev and 0.636±.01 ev respectively for the reorientation process of IV dipoles. This supports the theory of Dreyfus which predicts a decrease in ε with a decrease in impurity ion radius in the range of 0.9 to 1.4[subscript]°A. The expected decrease in relaxation time constant (T) with increasing temperature was observed to occur about 15°K higher in temperature than expected for the KC1:Pb²⁺ based on predictions from work reported in the literature. When samples of both KC1:Pb²⁺ and KC1:Cd²⁺ were heated in the presence of water vapor, changes in the relaxation properties were observed. For the KC1:Cd²⁺ system the interaction was very rapid and difficult to observe. Two separate relaxations were observed however in a sample which had been exposed to atmospheric water. The one occurring at lower temperatures had nearly the same properties as the DCM results for untreated samples. The data for KC1:Pb²⁺ were not separable into two definite processes, however; there were indications that the difference between the DCM and other work could be due to the presence of more than one species of dipole. It was concluded that differences in the pretreatment of the salt before crystal growth were responsible for these results and that the relaxation in the lower temperature range was due to the interaction of the impurity ion with OH⁻. The concentration of dipoles was increased by heating the samples and vacuum quenching in order to reverse the aggregation process and freeze the dipole concentration at an optimum value.
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