The phenomenon of the anomalous transmission of x rays in perfect crystals has been used to follow the perfection of dislocation‐free crystals of silicon heat‐treated at 1000°C. The heat treatment gives rise to impurity clustering or the formation of strain centers, which results in a decrease of x‐ray intensity of over two orders of magnitude. Upon heating at 1350°C, the clusters are dissolved and the original perfection of the crystal is restored. There is strong evidence that the impurity is oxygen, since the kinetics of the intensity decrease depends on the oxygen concentration of the crystals and is slowest for floating‐zone crystals, which presumably contain less than 5×1016/cm3 of oxygen. All of the x‐ray changes appear to occur before the infrared absorption band at 9 μ (which is a measure of the dissolved oxygen) begins to decrease. Thus the x‐ray anomalous transmission method appears to be measuring changes that occur in the early stages of impurity clustering. The anomalous transmission method should, therefore, be an important tool for the investigation of preprecipitation phenomena that may occur in crystals of high initial perfection.

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