According to my textbook:

In normal spinels $\ce{A^{II}}$ should have occupied octahedral voids, but this factor is outweighed by greater lattice energy or smaller cation $\ce{B^{III}}$.


Value of OSSE plays huge role in determining structure of spinels. In $\ce{A^{II}B2^{III}O4}$, if $\ce{A^{II}}$ has more LFSE gain in octahedral geometry than $\ce{B^{III}}$, then one's expected inverse spinel is formed. Normal spinel structure is assumed if both $\ce{A^{II}}$ and $\ce{B^{III}}$ are non-transition elements, since no LFSE is involved.

I don't understand why the author wrote the word expected. Aren't normal spinels expected more generally? Also, I had assumed that smaller ion (generally with higher positive charge) go to tetrahedral void and bigger ion to octahedral void. But here the situation is opposite, and it's simply stated that greater lattice energy is the dominating factor, is there better reasoning for the same? Also are there any exceptions where a non-transition element form inverse spinel?


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