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What is the oxidation state of the carbon ?

Dear Reader,

In the recent past the company which deals with gamma dosimeters where I work has changed from TLD (Thermal Luminescence Dosimeters) to OSLD (Optically Stimulated Luminescence Dosimeters) which work in a slight different way.

Being a chemist (a real chemist) I instantly wanted to know what was inside the lump of plastic which I was issued with. The old TLD badges contained lithium fluoride which absorbs energy from gamma rays and beta particles. The energy is stored in the crystals by displacing electrons from their rightful places, and when the solid is heated it emits light which is then measured. The emission of the light is used to estimate how much radiation the badge has been exposed to.

The supermodern dosimeters are slightly bigger, they seem like a clear giant plastic wine gum with a black bit of plastic inside with a disk thing in it. I sincerely hope that the inventor of the OSLD badge is not offended by my description of the object.

Not wanting to get into serious trouble for abusing a health and safety device I choose to look in the academic literature rather than banging the badge with a hammer to open it up. I found a paper which explained how these OSLD badges have a wider dynamic range and some other advantages. Also they contain alumina doped with carbon.

This Al2O3.C is aluminium oxide which has been doped with carbon. Now before we go any further I would like to point out that the history of optical devices based on doped aluminia is long and rich. For example the first laser was a ruby rod with a flash lamp. The ruby is a alumina which is doped with a little chromium. Now I imagine that a solid in which one +3 cation has been replaced with a trace of another +3 cation should be quite simple to understand.

Now before we go any further we need to understand what alumina is, it is infinite solid which contains aluminium cations in distorted octahedral with oxygens which are four coordinate. Now pure aluminium oxide is colourless and not active as an optical solid. But the doped versions can do things like storing the energy required for laser and OSLD operation.

Here is a ball and stick model of aluminia

Ball and stick view of alumina

Ball and stick view of alumina

While here is a spacefilling model

Space filling view of alumina

Space filling view of alumina

Now while aluminium forms +3 cations, carbon tends to be found in the +4 and +2 oxidation state in oxides. Now that is if we ignore carbon suboxide (C3O2) which is a real interesting organic compound which we will have to save for another day.

One option would be to have a solid which is Al(2-x)CxO3 which is doped with x electrons. This would be a n type semiconductor which has the occasional free electron in it. The classic semiconductor which it seems that most modern electronics is based on is silicon. To n dope silicon the best way is to include a small trace of phosphorus, while silicons have four electrons the phosphorus atoms have five. So after the four sigma bonds to the nearby silicons have been formed then one electron is left over. In case you are wondering what silicon looks like here are two lumps which were kindly donated to me by a friend in industry.

Opps I have not taken the photos yet

Well here is a picture of a unit cell of silicon.

Unit cell of silicon

Unit cell of silicon

The great problem is if we have C2O3.Al2O3 then if we assume that O has an oxidation state of -2 will have a oxidation state of +3. I think that the formula of the compound would be better written as C2O3(e)2.Al2O3. Now we can see that the carbon has an oxidation state of +4 which is more “normal”.

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