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BGO (Bismuth Germanium Oxide)

Dear Reader,

You might have noticed that I have been discussing some unit cells recently, most of these have been nice and friendly simple unit cells. I would now like to bite off something which is a bit bigger. Some months ago I meet some nuclear weapons inspectors from the UN, this pleasantly spoken group of men arrived at my place of work and then inspected the site. The wanted to visit every room and see what sort of things we were doing, to question the occupants of the rooms and also make some radiation measurements. They were equipped with the latest gadgets and gizmos, one of which was a portable gamma ray spectrometer. It was about the size of a VHS video cassette and I originally assumed it was a sodium iodide counter, the UN man corrected me and pointed out that it was a BGO based device.

BGO is a better crystal material for detecting gamma photons than sodium iodide, sadly it has a crystal structure which is a little confusing at first.

Here is the unit cell which looks like a very confusing group of spiders who are hugging each other, to understand why it is so much more complex than a sodium chloride or a calcium titanium oxide (CaTiO3) lattice we need to look at the metal centres in the solid.

The unit cell of bismuth germanium oxide

We will start with the easy one which is the germanium, the germanium has a tetrahedral coordination geometry which agrees perfectly with what we would expect from VSEPR if we assume that it has four sigma bonds. This is likely to be a germanium (IV) compound with a simple and well behaved germanium. It is rather easy so I will not bore you with the details of the coordination of the oxygens to the germanium.

The coordination environment of the germanium

Now for the interesting one, the bismuth. Here is one view which shows a hole in the first coordination sphere.

The inner coordination sphere of the bismuth showing a hole in the first coordination sphere

If we look at the coordination environment of the bismuth I hope that you will notice the hole in the coordination sphere. This is due to a special ligand which is known as the stereochemically active lone pair. Now lets go to a more simple thing with fewer electrons, it is ammonia. We should ask ourselves why is ammonia like a tripod rather than being a flat molecule ?

If we make ammonia flat then the electrons in the sigma bonds will be as far apart as possible, but if we look at the real life structure of ammonia both in the gas phase and in the solid (ammonia ice, see Boese, R.;Niederpruem, N.;Blaeser, D.;Maulitz, A.H.;Antipin, M.Yu.;Mallinson, P.R. Journal of Physical Chemistry, 1997, 101, 5794-5799). This might be like a clock which strikes 13 but please bear with me for a while.

The answer is that an electron pair which is not involved in bonding is present on the nitrogen of ammonia, it is a lone pair which can be thought of as the sigma bond to nowhere. For those of you who love 1980s references I suggest that you go off and sing the Talking Heads song “The Road to Nowhere” while for the rest of you lets get back on with the chemistry.

The electrons in the lone pair (bond to nowhere) are on average closer to the centre of the ammonia atom than the electrons in the sigma bonds to hydrogen so the lone pair is more able to repel the sigma bonds than they are able to repel each other. As a result the ammonia is distorted away from the perfect tetrahedral shape.

Back to bismuth, as we go down the p block groups the higher oxidation states become less stable and the lower oxidation states become more stable. For example

Boron and aluminium are almost never found in the +1 oxidation state, instead these elements are almost always in the +3 oxidation state. While thallium is very commonly found in the +1 oxidation state.

Also tin(II) is a reducing agent which is easy to change into tin(IV), while lead(IV) is a good oxidant which is easy to change into lead(II).

As a result of this trend the bismuth is very likely to be in the +3 state rather than the +5 state which phosphorus is often in. The change of phosphorus(III) to phosphorus(V) can be thought of as the two electrons from the lone pair being used to form two sigma bonds (or a sigma bond and a pi bond) to new atoms which are added to the phosphorus centre. It should be now clear to you that an increase in the oxidation state of a p block element by two results in the loss of a lone pair from the p block atom.

Lets have a series

Perchlorate ClO4 [Chlorine (VII)] has no lone pairs on the chlorine

Chlorate ClO3 [Chlorine (V)] has one lone pair on the chlorine

Chlorite ClO2 [Chlorine (III)] has two lone pairs on the chlorine

Hypochlorite ClO- [Chlorine (I)] has three lone pairs on the chlorine

Chloride Cl- [Chlorine (-I)] has four lone pairs on the chlorine

I hope by now you get the message that the lower the oxidation state the more lone pairs will be found on an atom.

I hope by now you will understand how the bismuth is less symmetric than the germanium, this lack of symmetry is likely to be the reason why the unit cell of BGO is so complex.

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One Response

  1. […] doing neither these polite men went on their way after collecting a gamma spectrum in my lab with a BGO detector, I strongly suspect that if I had been making bomb grade plutonium in the corner of my lab […]

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