In an ordered crystal structure there are always defects and one kind of defect is known as a imperfection. The image below represents imperfection in both occupational sights (a,c,d,e,h) and imperfections of arrangement (b,d,f,g). a is an interstitial atom that is small enough to exist between the lattice of crystal. b is an edge dislocation where a new row has forced the existing rows to part. c is called a self interstitial atom as it the same atom as the lattice but these are rare. d is a vacancy formed when the crystal cooled or perhaps diffused in from the surface and occasionally alpha bombardment will remove an atom, either way it is simply the lack of an atom in that place. e is a doped inclusion although an idealised one and it is forming f, a vacancy dislocation loop formed as vacancies and interstitials in their movements have a tendency of aggregating. Finally g is the alternative of f, an interstitial type dislocation loop and h is just an impurity formed by substituting an atom in the original crystal.
CeO2 is a very important oxide. It is often used in metal extraction due to its high reactivity but many other uses have been developed. It blocks out ultraviolet light but is transparent to the visible spectrum and is also non toxic. Being able to absorb the ultraviolet spectrum may give it a future as a catalyst in photochemistry but there is one more use for it as well. CeO2 also imitates the structure and dielectric properties of silicon and so could be used in the electronics industry as an situational replacement. Of course silicon, like all semiconductors, needs to be doped before being useful and this is the reason for the previous paragraph. A paper has been published recording the electronic and magnetic properties of pure, doped and defective CeO2. Carbon, nitrogen, phosphorous, silicon, lanthanum and praseodymium were all tested as dopants setting a firm structure that further research into CeO2 can be based on.