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Alum

Well I am considering growing a crystal of alum, before I start to grow my crystal I want to think about what a crystal is and what it is not.

Let us start with the nots

A crystal is not a mystic object with god like powers or supernatural abilities.

A crystal is not just any shape.

A crystal is not a fancy shape of glass

Now the positives

A crystal is a object made up of repeating units

A crystal is a nanoscale (or sub-nanoscale) diffraction grating for electrons, neutrons and X-ray photons.

I have chosen to write about alum because I have a packet of it, and I am planning on growing a crystal of it.

Alum is potassium aluminium sulphate is a white / colourless solid which forms nice crystals with great ease. Ginny Norman the school teacher accuses me of being a “real chemist”, so I better act like a real chemist so I thought about the arrangement of the atoms.

While world war two caused all manner of social unrest, weird (and nasty) politics and human suffering one very good thing did come out of it. The world of electronics got one almighty push forwards, things like high powered microwave sources, good VHF communications equipment and computer systems were created for the first time or were greatly improved during the war. It it interested to note that the classic compact power tetrode (4CX250B, http://www.cpii.com/docs/datasheets/213/4CX250B-7203.pdf) is used for many “FM” (88 to 108 MHz) radio stations is a world war two VHF/UHF radar valve made by companies such EIMAC (http://www.eimac.com/).

But back to crystals, the improvement in electronics during world war two lead to far better X-ray crystallography equipment. Computers have greatly improved the way in which the data is collected and the calculations which are done using the data.

VHF / UHF and microwave technology has allowed much brighter X-ray sources such as synchrotron to be made, this has allowed smaller and smaller crystals to be examined. It is important to bear in mind that a LINAC or cyclotron needs a very high voltage and high frequency RF source which to me looks like a high power VHF or UHF transmitter. The synchrotron is a cyclotron with some extra features to make it better.

As a result chemistry has obtained some new tools since world war two, these include NMR and modern X-ray crystallography. I looked at the unit cell of alum and first I was shocked by the number of atoms present.

Then I concentrated on the potassium only, I then noticed that the potassiums are present in the following locations in a cubic unit cell.

0.5 0.5 0.5

0.5 0 0

0 0.5 0

0 0 0.5

Then I added the aluminiums

The aluminiums are at the following locations

0 0 0

0.5 0.5 0

0.5 0 0.5

0 0.5 0.5

I will show you soon how to start to build up your model of the unit cell based on where the atoms are.

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One Response

  1. Ooooo smashing! We grew alum crystals earlier this year but it didn’t work too well, the atmosphere is too damp here to make good crystals, not even copper sulphate worked that well!

    I will point my students to this post for them to have a look!

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