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Spinels

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

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Spinels

  1. 1. - 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. 2. » 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. 3. » Bivalent ions occupy tetrahedral sites, trivalent ions occupy octahedral sites. » General Formula : » A+2B2 +3 O4 OR (AII)tet(BIII)2 octO4 » E.g Mn3O4 (i.e Mn+2Mn2 +3O4 ) » MgAl2O4 (known as spinel), ZnFe2O4, FeCr2O4 (chromite) etc.
  4. 4. » All bivalent cations and half the trivalent cations in octahedral holes. The other half of trivalent cations are in tetrahedral holes. » General Formula B+2B+3A+3O4 or (BII)oct(BIII )oct(AIII)tetO4
  5. 5. » 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. 6. » 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. 7. Config CFSE d4 d5 d6 d7 Oct (B+2) 6 0 4 6 Oct (B+3) 9 0 6 9 Tet (A+2) 4/9x4 0 4/9x6 4/9x12 Tet (A+3) 2/3x4 0 2/3x6 2/3x12
  8. 8. » Predict spinel structure for i. Mn3O4 ii. Fe3O4 iii. Co3O4

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