UPSC Combined Geo-Scientist, Mains · 2020 · Mains Paper I

Q1(a). Normal and inverse spinels

Question

Fe₃O₄ adopts inverse spinel structure while Co₃O₄ adopts normal spinel structure. Explain.

Fe₃O₄ is (Fe³⁺)ₜₑₜ[Fe²⁺Fe³⁺]_(oct)O₄; Co₃O₄ is (Co²⁺)ₜₑₜ[Co³⁺2]_(oct)O₄.

In a normal spinel AB₂O₄, divalent A occupies tetrahedral holes and trivalent B octahedral holes.
High-spin Fe(III), d⁵, has zero first-order CFSE in either geometry, while Fe(II), d⁶, has a preference for octahedral sites.
In Co₃O₄, Co(III) strongly favours low-spin octahedral d⁶=t_(2g)⁶, leaving Co(II) in tetrahedral sites. This yields the normal distribution.

The full model answer

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CoversNormalinverse spinelsCrystal structures