6)   Choose the most suitable crystal.

Remember, the crystal you choose largely determines how much effort you will have to expend later on. 
Good crystals can give good data or crummy data but lousy crystals will only ever give crummy data.

It usually takes a bit of practice to routinely pick the most appropriate sample. 
Some people get the hang of it almost immediately while others apparently never get it.

You are aiming to pick a crystal with sufficient volume, so blocky crystals tend to be better than needles.

blocky crystals
needle crystals
thin needles


Crystals come in all shapes and sizes.

A good size for organics with (say) all atoms lighter than sulphur would be a block about 0.25mm on a side, but this is just a guide. Some visually stunning crystals end up being much worse than crystals that are not so pretty - the golden rule is try them. Even if they are < 0.1 mm on a side, try them.

Large crystals with heavy atoms will cause absorption problems, e.g. when the x-ray beam is absorbed by different amounts for different reflections. The problem is that each reflection is attenuated differently, by some amount that is not known a priori. Although it is usually possible to correct for this to varying degrees, it is much better to pick a crystal that will minimize the problem to begin with.  Absorption corrections will be covered elsewhere.

Sometimes none of the crystals seem to be free of problems.  Do not despair - all the samples below eventually gave acceptable data - even the last one, but again, they all required surgery.  Cutting, breaking and cleaning are all important skills that you will have to learn.

usable_crystal_1 usable_crystal_2 usable_crystal_3 usable_crystal_4 usable_crystal_5 usable_crystal_6

The oil makes it easy to manipulate the crystals using a needle and to perform surgery because it holds the crystals steady while you work on them.

Transparent crystals can be checked between crossed polarizers. 

Swivel the polarizer on the very bottom of the microscope objective - see how the view goes light and dark.  With the polarizers crossed (dark) slowly turn the microscope stage.  See how the crystals go light and dark as you rotate them.

uncrossed polarizers
crossed polarizers
turned about 45º turned another 45º

This allows you to tell if your crystals are twinned. If your crystal appears to have two or more regions that disappear at different times or if it doesn't disappear at all, then you have a problem. Twinning is a complicated business that needs a whole separate document, just to cover the basics.

Opaque crystals usually require reflected light and can be viewed with either a white, black or some other background.

white background
black background
dull metal background
mounted crystal

At some point you will need to sketch and measure the crystal.  If you know that your crystals are stable you can do this now.  Otherwise you should measure it after data collection.

When the SZX12 microscope ZOOM is set to "25", then one small division in the eyepiece is a tenth of a millimetre. The SZX9 is a little different - on this microscope, with a ZOOM of "25", one tenth of a millimetre is 2.5 small divisions.

Feel free to play with the microscope and figure out what the scale is on other ZOOM settings.

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