Ligand field stabilization energies octahedral compared with tetrahedral

In the normal ABjO spinel structure, the A ions (Fe in this example) occupy tetrahedral sites and the ions (Cr in this example) occupy octahedral sites. The fact that FeCf204 exhibits the normal spinel structure can be understood by comparing the ligand-field stabilization energy of high-spin octahedral Fe and octahedral Cr. With six d electrons, LFSE = -0.4Ao for high-spin Fe. With only three d electrons, LFSE = -1.2Ao for Cr. Since the Cr ions experience more stabilization in octahedral sites than do the Fe ions, the normal spinel structure is more stable than the inverse spinel structure, which would exchange the positions of the Fe ions with half of the ions. 

---