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Electronic defects as reaction intermediates in sodium chloride films Adams, Richard James

Abstract

Evaporated films of radioactive sodium chloride have been prepared by direct sublimation onto a water cooled quartz substrate at 10-⁵ mm of mercury. These possess specific surfaces of from 30-100 m²/g and show remarkably high exchange reactivity to chlorine. From kinetic studies using ³⁶Cl incorporated in the solid it has been found that the extent of exchange C follows a fractional power of the time C = at[superscript n] and that the rate is independent of surface area, so that the possibility of the rate controlling step involving diffusion is ruled out. These features had been reported in an earlier study but required confirmation with a wider range of specific surface and a modified procedure to measure surface area before reaction. The major part of the work is designed to elucidate the role of electronic defects in the exchange mechanism from the pressure and temperature dependence of the exchange rate and from the effect of introducing electronic defects by X-irradiation or fluoridation. These latter processes cause the kinetics of the reaction to change completely to a second-order law, and provide strong evidence to support an earlier tentative suggestion that electronic defects are involved in the reaction, and that a process of adsorption of a chlorine molecule into a pair of defects is important. Detailed mechanisms are proposed for both the power law and the second-order reactions, largely on the basis of the pressure dependence. Both mechanisms use two species of electronic defect, corresponding to Seitz's models for V₂ and V₄ centres, and the "power law" mechanism requires a transition complex between the two defects. Measurements by X-ray diffraction on the particle size in the evaporated films has shown them to be in the range 250-500 Å, and an estimate of the strain from the same results suggests that roughly one dislocation per particle is present.

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