Space charge resulting from imperfections and effects thereof in ionic crystals

The equilibrium defect distribution is calculated for a pure ionic crystal of the type M+X=. It is shown that the normally assumed state of bulk electrical neutrality is a consequence of the presence of potential fields beneath the surfaces and around edge dislocations. These potential fields~ resulting from imperfection space charge~ modi,fy the innate free formation energies of the defects in a manJ:!ier such that bulk electrical neutrality exists at sufficiently high temperatures. At low temperatures it is shown that bulk electrical neutrality does not exist and effects due to the dislocation denSity are discussed. Calculations are given for crystals in which Schottky disorder predominates as well as for crystals in which Frenkel disorder predominates. Calculations of the equilibrium defect distribution for crystals containing divalent impurities are also given. Again it is shown, that, because of potential fields beneath the surfaces and around edge dislocations~ bulk electrical neutrality exists at sufficiently high temperatures. The influence of association of oppositely charged defects is described using a short range interaction. In addition the manner in which explicit account can be taken of the formally included long range interaction is outlined. The theory is applied in detail to NaCl. It is shown that edge dislocations» both in pure and doped crystals» develop,·a charge which is compensated electrically by a surrounding space charge. The charge on the dislocation is discussed with reference to recent work dealing with charged dislocations. The effect of the charge distribution on and around the dislocations on the mechanical properties of ionic crystals is discussed. Explanations for the Joffe effect and the brittleness of ionic crystals are proposed. The presence of the potential field at a surface is discussed with reference to the electron affinity of ionic crystals. Finally» an experiment is proposed whereby the potential difference across the surface potential field can be measured directly.U of I OnlyThesi