Different types of defects in ionic structures may interact with each other strongly. These interactions usually result in drastic changes in the properties of the material. One such interaction is the so-called space-charge region between the point defects and surfaces/interfaces. What is a space-charge region? Use MgO as an example and tell us how and why a space charge region develops in an oxide which is known to have intrinsic type defects. What kind of surface do you expect to develop in MgO, why? What kind of surface charge do you expect to develop in a fluorite structure ionic oxide, why? Use diagrams to show the spatial distribution of defects in MgO. What happens when the MgO is doped with Al³⁺ extrinsic defects. Write down the defect equation and show how the space charge region changes.