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FeO crystallizes in the rock salt structure with O²- in the FCC positions and Fe²+ in the octahedral voids. Due to a Schottky-type defect, the actual composition is Fe0.75O. Assuming charge neutrality is maintained by some Fe²+ being oxidized to Fe³+, find the number of effective Fe²+ ions per unit cell.

1. 1
2. 2
3. 3
4. 4

Correct answer: 1

Solution

Rock salt unit cell: 4 O²- at FCC positions and 4 cation sites at octahedral voids. In defective Fe0.75O: per 1 O²- there are 0.75 Fe ions. Per unit cell (4 O²-): total Fe = 3. Of these, some are Fe²+ (n2) and some are Fe³+ (n3) with one Fe site vacant per unit cell. Charge balance: (n2)(+2) + (n3)(+3) = (4)(2) = 8 (to neutralize 4 O²-). System: n2 + n3 = 3 and 2*n2 + 3*n3 = 8. Solving: n3 = 2, n2 = 1. Effective Fe²+ per unit cell = 1.

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