In a non stoichiometric sample of ferrous oxide with NaCl structure, the ratio of Fe + 3 to Fe 2 + was found to be 0.15 . The fraction of octahedral sites unoccupied is :

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FeO with NaCl structure means O²⁻ occupy fcc and Fe²⁺ occupy OHV.

$\frac{F e ^{3 +}}{F e ^{2 +}} = \frac{0.15}{1} = \frac{3}{20}$ ,Charge of Fe³⁺ = $3 \times 3 = 9$ .

Number of Fe²⁺ replaced = $9/2 = 4.5$ .

Out of 4.5 Fe²⁺ (OHV), 3 are occupied by Fe³⁺.

Unoccupied OHV sites = $4.5 - 3 = 1.5$ .

Total number of available OHV = $20 + 4.5 = 24.5$ .

Fraction of octahedral sites occupied is $1.5/24.5 \approx 0.062$ .

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