Spinels

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Spinel Structure
  • Formulae – (A2+)(B3+)2O4 or AB2O4 or AO.B2O3
  • FCC Packing of anions
  • Partial occupancy of both tetrahedral and octahedral sites i.e.1/8th of tetrahedral and ½ of the octahedral sites are occupied.
  • A spinel unit-cell is made up of eight FCC cells made by oxygen ions in the configuration 2×2×2, so it is a big structure consisting of 32 oxygen atoms, 8 A atoms and 16 B atoms.
  • Depending on how cations occupy different interstices, spinel structure can be Normal or Inverse.
       1.7.4.1 Normal Spinel
  • Chemical formula: (A2+)(B3+)O4
  • Examples are many aluminates such as MgAl2O4, FeAl2O4, CoAl2O4 and a few ferrites such as ZnFe2O4 and CdFe2O4.
  • In this structure, all the A2+ ions occupy the tetrahedral sites and all the B3+ ions occupy the octahedral sites.
  • Apply bond strength rule to verify the stoichiometry
  • Cations: - A2+ - 2⁄4 ; B3+ - 3⁄6
    Oxygen valence = (2⁄4x1)+ (3⁄6x3) = 2
    Figure 1.30 Schematic of spinel structure
       1.7.4.2 Inverse Spinel   B(AB)O4
  • Chemical formula: (A2+)(B3+)2O4 but can be more conveniently written as B(AB)O4.
  • Most ferrite follow this structure such as Fe3O4 (or FeO.Fe2O3), NiFe2O4, CoFe2O4 etc.
  • In this structure, ½ of the B3+ ions occupy the tetrahedral sites and remaining ½ B3+ and all A2+ions occupy the octahedral sites (now you can hopefully make sense of the formula in the previous line).
 
 
 
 
 
 
 
 
 
 
 
 
 
 

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 Mn3O4  exist as Mn.Mn2O4, therefore Mn exists in +2 and +3 oxidation state.



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




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








CFSE of Mn+3 in
 octahedral feild = -0.6∆0
CFSE of Mn+3 in
 tetrahedral feild = -0.4∆t
 

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

 

 

 

 

 

  Rules:

  1. If both  A+2 and  B+3 are non transition metals then spinel will be NORMAL.
  2. If A+2  is Non transition metal and B+3 is transition metal (having d1 d2 d3 d4 d6 d7 d8 d9) then spinel will be NORMAL.
  3. If is transition metal with (d1 d2 d3 d4 d6 d7 d8 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 d4system 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 (Al0.75Ni0.25)tet [Ni0.75Al1.25]octaO4  is a Random Defected Inverse Spinel.The CFSE of NiII 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 (Al0.75Ni0.25)tet [Ni0.75Al1.25]octaO4.

    •  FeAl2O4 is Normal Spinel because  has stronger preference for Oh sites due to its High Lattice Energy 


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