Chemistry · Ch 6 — Solid State
Metal Deficiency Defect
Metal Deficiency Defect
Metal deficiency defect arises when a crystal contains fewer cations than the number of anions would normally require. This defect is only possible in crystals whose cation can exist in more than one oxidation state, because it is exactly that variability that lets the crystal compensate for the missing charge.
Worked example -- FeO. In an FeO crystal, some of the Fe2+ ions are found to be missing from the lattice. To keep the crystal electrically neutral despite this, TWICE as many of the remaining Fe2+ ions are oxidised to Fe3+ for every Fe2+ that went missing (since going from +2 to +3 supplies one extra unit of positive charge per ion oxidised, and 2 such oxidations exactly balance the 2 units of positive charge lost when 1 Fe2+ ion vacates its site). As a result, the combined total of Fe2+ and Fe3+ ions in the crystal ends up smaller than the number of O2- ions present. This h …
What this figure shows. A grid of large green spheres (O2-) with smaller pink spheres (Fe2+) and a few orange spheres (Fe3+) tucked between them; two lattice positions are marked with a dashed circle showing a cation site left vacant (missing Fe2+), with the surrounding orange Fe3+ spheres representing the compensating oxidation of extra Fe2+ ions; a legend keys green to O2-, pink to Fe2+ and orange to Fe3+, captioned 'Metal Deficie …