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General Chemistry Principles, Patterns, and Applications, 2011

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If a cation or an anion is simply<br />

missing, leaving a vacant site in an ionic crystal, then for the crystal to be electrically neutral, there must be a<br />

corresponding vacancy of the ion with the opposite charge somewhere in the crystal. In compounds such as KCl, the<br />

charges are equal but opposite, so one anion vacancy is sufficient to compensate for each cation vacancy. In<br />

compounds such as CaCl2, however, two Cl − anion sites must be vacant to compensate for each missing Ca 2+ cation.<br />

These pairs (or sets) of vacancies are called Schottky defects <strong>and</strong> are particularly common in simple alkali metal<br />

halides such as KCl (part (a) in ). Many microwave diodes, which are devices that allow a current to flow in a single<br />

direction, are composed of materials with Schottky defects.<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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