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In a spinel structure, what is the minimum internuclear distance between an Fe2+ ion and an Fe3+ ion?

1. sqrt(3)*a/4
2. a/2
3. a/sqrt(2)
4. a

Correct answer: sqrt(3)*a/4

Solution

In spinel structure (like Fe3O4 = FeO*Fe2O3), Fe2+ occupies tetrahedral voids and Fe3+ occupies octahedral voids (normal spinel). The minimum distance between a tetrahedral void and the nearest octahedral void in an FCC arrangement is sqrt(3)*a/4, where a is the unit cell parameter.

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