Lecture 10 Point Defects

Lecture 10 Point Defects - Substitutional Defects Pick up...

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Substitutional Defects Pick up from Wednesday. What matters is a) Size – obviously large in small is not good b) Chemical compatibility, i.e. energy levels & bonding similar. For instance Ag & Au are rather similar both in size and chemistry so the energies of substituting them into a matrix will be similar. While both K and Ag are monovalent (Ag can be 2+ but not often) K is much more electropositive (E-levels higher) so energies of substitution will be different. Aside. Most compounds have some levels of impurities. These may be intentional or undesirable (e.g. Ca in most MgO). Often they do not behave as desired without specific levels of impurities for specific functionality, e.g. fuel cells (ask Megna) or catalysts. Beyond substituting an element with the same valence, one can introduce something with a different valence. For instance, add a little Na or Al into MgO. There are two ways this can be done: a) Straight substitution. In this case since Na+ replaces Mg2+ this in effect adds an electron into the conduction band; similarly replacing by Al3+ adds a hole into the valence band. b) Compensated substitution, for instance for every Na+ one removes ¼ O 2 (each O is 2-) i.e. ½ an O vacancy per Na. Similarly for every Al3+ added one would have a Mg
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Lecture 10 Point Defects - Substitutional Defects Pick up...

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