Chemistry · Ch 6 — Solid State
Metal Excess Defect
Metal Excess Defect
Metal excess defect arises when a crystal contains more metal ions (cations) than anions. It is seen in the alkali metal halides, such as NaCl and KCl. Electrical neutrality is restored in one of two ways: either by anionic vacancies exactly equal in number to the excess metal ions, or by extra cations sitting at interstitial positions, each paired with an extra electron also trapped interstitially.
Worked example -- NaCl heated in sodium vapour. When NaCl crystals are heated in the presence of sodium vapour, Na+ ions form and deposit on the crystal's surface. To balance this, Cl- ions diffuse out to the surface from their lattice points and combine with the new Na+ ions. The electron that the sodium atom lost in becoming Na+ diffuses INTO the crystal lattice instead, where it occupies the vacancy left behind by the departed Cl- ion. Such an anionic vacancy occupied by an unpaired electron is called an F-centre (from the German 'Farbe', colour) -- and it is exactly these trapped electrons, absorbing visible light, that give the resulting non-stoichiometric crystal, written Na(1+x)Cl, its distinctive yellow colour. …
What this figure shows. A dense packing of large green spheres (Cl-) with small pink dots (Na+) tucked between them, and one lattice site near the centre marked by a dashed white circle containing 'e-' with an arrow labelling it 'F center' -- showing an anionic vacancy occupied by an unpaired electron in place of a Cl- ion; a legend keys pink to Na+ and green to Cl-, captioned 'Metal Excess Defect'. Unlike a genuinely empty vacancy, the dashed circle here is drawn containing a trapped electron symbol rather than being left blank -- this is the visual distinction between an ordinary missing-ion vacancy and an F-centre, and it is specifically this trapped electron that absorbs visible light and produces the yellow colour of …