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Chromium oxides

Dear Reader,

I have no idea why my blog came up when someone was searching for chromium oxides, but perhaps I should treat this as a request for me to comment on the chemistry of chromium and related elements.

Before we get going any further it is a good idea to think about the structure of chromium metal, it is a cubic solid with a cell which has sides which are 2.88494 Å long and the chromium atoms are found at the following fractional coordinates

0.0 0.0 0.0

0.5 0.5 0.5

I am sure that you lot can make your own model of the unit cell of the chromium, but I will relent and draw the other solids for you.

The first oxide is chromium(II) oxide, this has a rock salt like lattice which is cubic and the sides of the cell are 4.16 Å long. It is important to make an estimate of the volume change which occurs when the metal is converted into the oxide. I will explain layer why this is important.

The oxygen atoms are at the following fractional coordinates

0.5 0.0 0.0

0.0 0.5 0.0

0.0 0.0 0.5

0.5 0.5 0.5

The chromium atoms are at the following sites

0.0 0.0 0.0

0.5 0.5 0.0

0.5 0.0 0.5

0.0 0.5 0.5

Here is the solid.

Chromium(II) oxide

I looked at the cell of chromium(III) oxide and it is very complex, I will not make an attempt to discuss how to draw it. If you want to draw it then please be my guest. The key thing to note here is that the chromium(III) centres have octahedral coordination environments.

Here is the unit cell.

Chromium(III) oxide

I looked in the inorganic database and I found that chromium(IV) oxide has a tetragonal cell with the following dimensions.

4.419 Å

4.419 Å

2.915 Å

A tetragonal cell is like a cubic cell except it has been squashed, one axis will be a different length to the others. On close examination of the cell it looked like the cell for ruthenium dioxide which is an example of how the same structures appear again and again in inorganic chemistry.

The chromium atoms are at the following fractional coordinates

0.0 0.0 0.0

0.5 0.5 0.5

The oxygen atoms are at the following fractional coordinates

0.3026 0.3026 0.0000

0.6974 0.6974 0.0000

0.1974 0.8026 0.5000

0.8026 0.1974 0.5000

Here is the unit cell

Chromium dioxide AKA chromium(IV) oxide

I could not resist trying out the chrome finish in POVray for the chromium atoms, but it did not turn out well. Here is the picture.

Chromium dioxide with the not so good colour scheme

The next one in our series is chromium trioxide, this has a boxy cell which has three different lengths but thankfully all the angles are ninety degrees.

Chromium trioxide AKA chromium(VI) oxide

Now we will make use of the density measurements to try to make sense of things, The density of chromium metal and the oxides are shown in the following table. Using the relative atomic masses of oxygen (16) and chromium (52) we can make some useful calculations about corrosion.

Species Density

Percentage
Cr (% w/w)

Chromium
oxidation state

Electronic
state

Pilling-Bedworth ratio

           

Cr

7.2

100

0

s2
d4

1.00

CrO

6.27

76

II

s0
d4

1.50

Cr2O3

5.22

68

III

s0
d3

2.02

CrO2

4.89

62

VI

s0
d2

2.38

CrO3

2.82

52

VIII

s0
d0

4.91

 

The P-B ratio is the ratio of the volume of the oxide to the metal, it should be clear to you that chromium oxide has a much larger volume than the metal. If chromium metal is heated in air the oxide formed will not be able to stick to the surface the expansion will tend to cause the oxide to spall off, the volume change for the conversion of aluminium to aluminium oxide has a P-B ratio of only 1.28 so when the oxide forms the mechanical stresses on the new oxide coat are less likely to make it spall off. The fact that the P-B ratio is slightly larger than 1 makes the oxide formation seal any cracks in the surface of the metal.

For the formation of calcium oxide the P-B ratio is 0.64. The fact that the P-B ratio is below one causes the oxide to have a smaller volume than the metal, again this imposes mechanical stresses on the system as the oxide forms. So the oxide offers little or no protection to the metal.

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3 Responses

  1. Dear Mark, Could you comment on the probability that CrO could be formed while doing solid state reduction of chromite ore with coal as reductant at temperatures around 1300C. Of course there is excess carbon available.

    Thanks, Louis

    • Regarding chromium monoxide, I think you need to check the Gibbs free energy of formation of this compound at different temps, then you will be able to work out if it can form under a given set of conditions with a reducing agent. The technical name of the graph you need is an Ellingham diagram

  2. Dear Mark,
    What is the reference of CrO lattice constant? I am currently studying CrO magnetic properties using Density Functional Theory. However, I did not find a good reference of CrO in any online source.

    Thanks,
    Shun

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