Chemistry · Ch 6 — Solid State
Impurity Defect
Impurity Defect
A general and deliberate way of introducing point defects into an ionic solid is by adding impurity ions. If the impurity ion's valence (charge) differs from that of the host ion it replaces, vacancies must appear in the host lattice to keep the crystal electrically neutral.
Worked example -- CdCl2 in AgCl. Adding CdCl2 to AgCl produces a solid solution in which the divalent Cd2+ cation takes up a lattice position normally occupied by the monovalent Ag+ cation. Because Cd2+ carries twice the charge of the Ag+ it replaces, this would upset the crystal's electrical neutrality if nothing else changed -- so, to compensate, a proportional number of Ag+ ions leave the lattice altogether. Each departed Ag+ ion leaves behind a cation vacancy -- and it is …
Worked out. A 'Do You Know?' box explaining piezoelectricity (the piezoelectric effect) as the appearance of an electrical potential across the sides of a crystal when it is subjected to mechanical stress -- the word literally meaning 'electricity resulting from pressure' -- and noting that the inverse piezoelectric effect (mechanical deformation on applying a voltage) also exists. It poses the open question of whether pressing many piezoelectric crystals repeatedly, or burying them under streets to capture the mechanical energy of passing vehicles, could harvest a genuinely useful amount of electricity -- an idea known as energy harvesting, already used in practice in power-generating footwear with piezoelectric inso …