Pick-up and loss of charge from dislocations in Mn⁺⁺-doped sodium chloride crystals

Abstract

The technique of Whitworth (1967) has been used to determine the charge per unit length on edge dislocations in NaCl crystals containing 116 p.p.m. of Mn⁺⁺ ions. In crystals rapidly cooled from 400°c to keep many of the Mn⁺⁺ ions and associated vacancies in solution, the dislocation charge increases with increasing amplitude of dislocation motion, and negative charges as large as one ionic charge per two cation sites on the dislocation core have been observed. When the dislocations are left at rest in the crystal for several hours their charges decrease and they become pinned. In contrast the dislocation charge does not increase appreciably when dislocations are moved in as-cleaved crystals in which the Mn⁺⁺ ions and vacancies are mainly precipitated. These results are discussed in terms of the sweep-up of positive-ion vacancies, and the subsequent diffusion of these vacancies off the dislocation core and of Mn⁺⁺ ions onto it.