What is the Difference Between Spinel and Inverse Spinel Structure?

The main difference between spinel and inverse spinel structures lies in the distribution of cations in their crystal lattices:

• Spinel structure: In this structure, A cations occupy 1/8th of the tetrahedral sites, while B cations occupy half of the octahedral sites. The general formula for a spinel structure is A[subscript{8}][A[subscript{16}]O[subscript{32}].
• Inverse spinel structure: In this structure, all A cations and half of the B cations occupy octahedral sites, while the other half of the B cations occupy tetrahedral sites. The general formula for an inverse spinel structure is B[subscript{1/2}[A[subscript{1/4}][B[subscript{1/4]]O[subscript{4].

Inverse spinel structures are formed when the divalent A[superscript{II}] ion has a comparatively more favorable crystal field stabilization energy (CFSE) gain in octahedral geometry than the trivalent B[superscript{III}] ion. The choice of the normal or inverse spinel structure is primarily driven by the CFSE of the transition metal ions.

Examples of spinel and inverse spinel structures include:

• Normal spinel: MgAl[superscript{2+}]Osubscript{4.
• Inverse spinel: Fe[superscript{3+}]Osubscript{4.

Intermediate cases between normal and inverse spinel structures also exist, where the cation distribution can be described as (A[subscript{1-x}][B[subscript{x}][A[subscript{x/2}][B[subscript{1-x/2]][O[subscript{4].

Comparative Table: Spinel vs Inverse Spinel Structure

The key difference between spinel and inverse spinel structures lies in the arrangement of cations in their unit cells. Here is a comparison of the two structures:

Spinel structures have a close-packed array of anions, where A-site cations fill 1/8 of the tetrahedral holes and B-site cations fill 1/2 of the octahedral holes. In contrast, inverse spinel structures have all A cations and half of the B cations occupying octahedral sites, and the other half of B cations occupying tetrahedral sites. The choice between the normal and inverse spinel structures is primarily driven by the crystal field stabilization energy (CFSE) of the cations.