Spinel group

Spinels are chemical compounds of the general type AB2X4, wherein A, B are metal cations whose oxidation number, the sum 8, and X is predominantly a divalent oxygen or sulfur anion (ie, a total of the compound is an oxide or sulphide) is. Important examples are the magnesium spinel or spinel -called name to the class of compounds ( MgAl2O4 ) and gahnite ( ZnAl2O4 ).

Use

The cobalt - cobalt aluminate spinel CoAl2O4 ( Thénards blue ) is known as a pigment in industry and in the classical analytical chemistry test reactions. Rinmans Green has been mistakenly often referred to as spinel, * (CoO ) * x (x = 5% ) is in reality but as a mixed oxide present in the composition ZnO. The spinel ZnCo2O4 is black.

Similar to garnet it is also used as a microwave ferrites, however, has higher losses.

Varieties

  • Aluminum spinels: spinel
  • Hercynite
  • Gahnite
  • Cobaltaluminiumspinell ( Thénards Blue)
  • Iron ( III) spinels: lodestone
  • Magnesioferrit
  • Franklinite
  • Pleonaste
  • Chromium spinels: chromite
  • Magnesiochromite
  • Picotite
  • Titanium spinel: Ulvit
  • Cobalt spinels: Cobaltaluminiumspinell CoAl2O4 ( Thénards Blue)
  • Cobalt Black Co3O4

Crystal structure

Many compounds of the type AB2O4 crystallize in the spinel type crystal structure, which is one of the most important and common structural types and is referred to by the main mineral as a spinel structure.

The O2 - ions thereby form a cubic- dense crystal lattice, the tetrahedral holes as Al3 are occupied to one-eighth of mostly dipositive A ions such as Mg2 and the octahedral voids half of mostly triply positively charged B ions. Other charges are also possible, e.g. W6 ( Na 2) O4

Inverse spinels have AB2O4 the formula. They are present in the same grid. However, the A ions occupy one quarter of the octahedral sites, the B ions quarter of the octahedral and one-eighth of the tetrahedral holes. Thus, the octahedral sites and half the tetrahedral holes are a total re-occupied to one-eighth. Examples include magnetite Fe3O4 ( = Fe (III) 2Fe (II)) and TiMg2O4.

Calculation

It is possible to predict whether a spinel is normal or inverted. Here we compare the ligand field stabilization energy ( LFSE ) in the normal spinel with the LFSE in the inverse spinel.

Example

FeCr2O4:

  • Fe2 : Tetrahedral site: In the tetrahedral ligand field, the three t2 orbitals are raised and lowered by 4 Dq 2 s orbitals by 6 Dq. These are 6 electrons filled ( Fe2 is a d6 ion ). Thus the LFSE is
  • Octahedral: In the octahedral ligand field the two eg orbitals are raised and lowered by 6 Dq the three t2g orbitals by 4 Dq. These are filled with six electrons in the high-spin configuration. Thus the LFSE is
  • Cr3 :
  • Tetrahedral site: Cr3 is a d3 ion. Thus the LFSE is
  • Octahedral: The LFSE is

Normal spinel ( FeTCrOCrOO4 ):

Inverse spinel ( FeOCrTCrOO4 ):

Thus, the normal spinel has a higher ligand field stabilization energy. FeCr2O4 is present as a normal spinel.

Magnesioferrite Ligand field theory
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