Spinels

Spinels, mixed metal oxides, explained in simple terms. Includes calculation method for CFSE and prediction for normal and inverse spinel.

1. SPINELS Mixed metal oxides. Cubic close packed system, cubic sites occupied by oxide ligand, which are weak field ligands. Contain tetrahedral and octahedral holes Based on type of metal atom occupying them, they are classified into two types i . Normal Spinels Ii. Inverse Spinels or antispinels

2. NOTATIONS A +n represents a metal atom of charge +n occupying a tetrahedral site. B +m represents a metal atom of charge +m occupying an octahedral site.

3. Normal Spinels Bivalent ions occupy tetrahedral sites, trivalent ions occupy octahedral sites. General Formula : A +2 B 2 +3 O 4 OR (A II ) tet (B III ) 2 oct O 4   E.g Mn 3 O 4 (i.e Mn +2 Mn 2 +3 O 4 ) MgAl 2 O 4   (known as spinel ), ZnFe 2 O 4 , FeCr 2 O 4  (chromite) etc.

4. Inverse Spinels All bivalent cations and half the trivalent cations in octahedral holes. The other half of trivalent cations are in tetrahedral holes. General Formula B +2 B +3 A +3 O 4 or ( B II ) oct (B III ) oct (A III ) tet O 4  

5. Prediction of Spinel Structures Since Oxide ions are weak field ligands, the cations are found in high spin configurations. Finding out whether a given spinel is normal or inverse, can be done by calculating CFSE for both cases. Greater the CFSE, more stable the structure and hence the spinel takes that structure.

6. CFSE Values Value of 10Dq for a trivalent cation is 1.5 times more than that of a bivalent ion of the same element. Value of 10Dq is reduced by a factor of 4/9 on going from an octahedral (B) site to a tetrahedral site.

7. Config CFSE d 4 d 5 d 6 d 7 Oct (B +2 ) 6 4 6 Oct (B +3 ) 9 6 9 Tet (A +2 ) 4/9x4 4/9x6 4/9x12 Tet (A +3 ) 2/3x4 2/3x6 2/3x12

8. Predicta Predict spinel structure for Mn 3 O 4 Fe 3 O 4 Co 3 O 4