Enhanced Thermal Expansion in X-Rayed Rocksalt

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Abstract
The volume expansion of NaCl heavily irradiated at room temperature with penetrating x rays has been studied with a photoelastic technique. In the course of the study a new effect was found; the coefficient of thermal expansion of NaCl is increased by irradiation. The magnitude of the fractional increase in expansion coefficient is typically somewhat less than 1%, and is about 14 times the concentration of radiation-induced defects, as measured by the volume expansion. The thermal annealing, above room temperature, of the enhancement of expansion coefficient has been observed; within experimental error the effect anneals concurrently with the volume expansion. The increase in expansion coefficient seems to be correlated only with the total concentration of lattice point defects, and is rather insensitive to their electronic state and spatial distribution.