SPINELS (Normal or Inverse)

Spinel is a name of Mixed Ore of Magnesium(Mg) and Aluminium(Al), its molecular formula is MgAlO₄(MgO.Al₂O₃). Here Mg is in +2 oxid. state and Al is in +3 oxid. state.

Crystal Structure of Fe₃O₄

So, in general we can say Spinels are AB₂X₄type complexes

where, A= any dipositive metal (eg. Mg⁺², Ni⁺²)

B= any tripositive metal (eg. Fe⁺³, Al⁺³, La⁺³)

X = O, S, Se etc.

General Properties Of Spinels

Spinels aremixed oxides having formula AO.B₂O₃
They have Face Centred Cubic (CCP) structures.
Total No. of Atoms per unit cell = 4

No. of Tetrahedral (Td) voids per unit cell = 8
No. of Octahedral (Oh) voids per unit cell = 4

TYPES OF SPINELS:

On the basis of position occupied by A⁺² ions and B⁺³ ions, spinels are of two types:

Normal Spinel:

The divalent A⁺² ions occupy the tetrahedral voids, whereas the trivalent B⁺³ ions occupy the octahedral voids in the close packed arrangement of oxide ions. A normal spinel can be represented as: (A⁺²)^tet(B⁺³)₂^octO₄
e.g. Mn₃O₄, ZnFe₂O₄, FeCr₂O₄

Inverse Spinel:

The A⁺² ions occupy the octahedral voids, whereas half of B⁺³ ions occupy the tetrahedral voids.

It can be represented as: (B⁺³)^tet(A⁺²B⁺³)^octO₄ E.g. Fe₃O₄ (ferrite), CoFe₂O₄, NiFe₂O₄ etc.

Prediction Of Structure Of Spinels:

basic rule for determining whether a complex is Normal or Inverse Spinel

For Normal Spinel:

CFSE of B⁺³ ions in Octahedral Feild > CFSE of B⁺³ ions in Tetrahedral Feild

>CFSE of A⁺² ions in Octahedral Feild
>CFSE of A⁺² ions in Tetrahedral Feild

[i.e. if CFSE of B⁺³ ions in Octahedral Feild is largest of all CFSE]

For Inverse Spinel:

CFSE of A⁺² ions in Octahedral Feild > CFSE of A⁺² ions in Tetrahedral Feild

>CFSE of B⁺³ ions in Octahedral Feild
>CFSE of B⁺³ ions in Tetrahedral Feild

[i.e. if CFSE of A⁺² ions in Octahedral Feild is largest of all CFSE]

Let's take an example of Mn₃O₄

Here Mn₃O₄exist as Mn.Mn₂O_4,therefore Mn exists in +2 and +3 oxidation state.

CFSE of Mn⁺²in
octahedral feild = 0∆_o

CFSE of Mn⁺²in
tetrahedral feild = 0∆_t

CFSE of Mn⁺³in
octahedral feild = -0.6∆₀

CFSE of Mn⁺³in
tetrahedral feild = -0.4∆_t

here CFSE of Mn⁺³ in Oh feild is more than all, therefore is Mn₃O₄ Normal Spinel

Rules:

If both A⁺² and B⁺³ are non transition metals then spinel will be NORMAL.
If A⁺² is Non transition metal and B⁺³ is transition metal (having d¹ d² d³ d⁴d⁶ d⁷d⁸ d⁹) then spinel will be NORMAL.
If is transition metal with (d¹ d² d³ d⁴d⁶ d⁷d⁸ d⁹ ) and B is also transition metal with (d⁰ d⁵ and d¹⁰) system then spinel will be INVERSE.

EXAMPLES FOR SPINEL AND INVERSE SPINEL STRUCTURES

1) MgAl₂O₄ is a normal spinel since both the divalent and trivalent ions are non transition metal ions. There is no question of CFSE.

2) Mn₃O₄ is a normal spinel since the Mn²⁺ ion is a high spin d⁵ system with zero LFSE. Whereas, Mn³⁺ ion is a high spin d⁴ system with considerable LFSE.

3) Fe₃O₄ is an inverse spinel since the Fe(III) ion is a high spin d⁵ system with zero CFSE. Whereas the divalent Fe(II) is a high spin d⁶ system with more CFSE.

4) NiFe₂O₄ is again an inverse spinel since the divalent Ni²⁺ (a d⁸ ion) has more CFSE than the trivalent Fe³⁺ (a d⁵ ion).

5) FeCr₂O₄ is a normal spinel since the divalent Fe²⁺ is a high spin d⁶ ion with CFSE = 4 Dq and the trivalent Cr³⁺ is a high spin d³ ion with CFSE = 12 Dq. Hence Cr³⁺ gets more OSSE while occupying octahedral sites.

6) Co₃O₄ is a normal spinel. Even in the presence of weak field oxo ligands, the Co³⁺ is a low spin d⁶ ion with very high CFSE. It is due to high charge on Co³⁺. Hence all the Co³⁺ ions occupy the octahedral sites.

Exception:

NiAl₂O₄ for which the formula can be written as (Al_0.75Ni_0.25)^tet [Ni_0.75Al_1.25]^octaO₄ is a Random Defected Inverse Spinel. The CFSE of Ni^II is greater in octahedral than tetrahedral coordination. But Al³⁺ also has strong preference for octahedral sites due to high lattice energy. This leads to almost complete randomization of all the cations on all the available sites. Its formula can be written as (Al_0.75Ni_0.25)^tet [Ni_0.75Al_1.25]^octaO₄.