The Best Crystal

The Best Crystal to use in data collection will: For a sample of 2/µ values, click here

Important Note

Sample Size

Bigger crystals always give better data than small crystals (when their qualities are comparable!), except when there will be absorption problem caused by some heavy elements (such as bromine, iodine and many metals). When the absorption is expected to be serious, the optimum size will be about 3/µ, where µ is the absorption coefficient (which depends on the cell volume and cell contents) [Ref: IUCr Texts on Crystallography 2: Fundamentals of Crystallography, Edited by C.Giacovazzo, 1992, pp. 304-5]. If we use smaller crystal, the data will be weak and poor quality. If we use bigger crystal, the absorption will cause serious error in the data. But for organic crystals, most of the time the size limit is the limit of the beam size (for Molybdenum X-ray; we commonly use the beam size of 1.0 mm). So it is advisable to make some effort to get bigger crystals, if good publication quality structure is desired (if the crystal is too big, we can always cut, but if it is too small, we can not do much).

S.Rühl and M.Bolte write in Z. Kristallogr., 215, 499-509 (2000): "The examinations were conducted with a crystal of 3 of a needle-like shape with dimensions 0.54×0.60×2.70 mm. The longest axis was successively cut to 1.60 mm, 0.90 mm and 0.54 mm. ... collimeter radius 0.8 mm ..."
"Surprisingly, the best results were obtained for the larger crystals [in terms of R(int), R(sigma), R1 and wR2; the drawback is that the exposure time was kept constant]. If the crystal is larger than the collimeter radius, a different proportion of the crystal is bathed in the X-ray beam depending on the angular position and therefore the intensity of the reflection differs due to the proportion of the crystal being in the X-ray beam. Obviously, this effect shows less stronger tendency towards a deterioration than expected. The effect of higher number of scattering centres and therefore the higher scattering power seems to overcompensate the negative expectations. Another explanation for worse data sets, when the crystal size had been reduced [by cutting/breaking], might be that the specimen suffered from being cut or suffered from radiation damages."
"... the scattering power is proportional to the volume of the crystal while the absorption is proportional to the thickness. Thus, as a rule of thumb, the optimal thickness can be calculated as about the twofold value of the reciprocal absorption coefficient [2/µ]."

The above article was noted from the free email abstract service and obtained by the courtesy of Zeitschrift für Kristallographie

Growing Crystals

So, we need crystals of the size approximately 0.6 mm maximum in each side. However, some compounds will crystallize as needles or plates in some solvents and in that cases, we have to try to grow the thinnest side as big as possible for easy handling (it may be only 0.1 mm or less!). We can use needles upto 1.1 mm in length or plates upto 0.75 mm in each side.

If the crystals are well packed or have heavy atoms (such as Iodine or heavy metals), then we will be able to use crystals of size just 0.1 mm in each side.

You can use any of the following techniques to get single crystals:

1. An outline of one simple crystallization technique.
2. Some general techniques of growing crystals.

Created by A.Chandrasekaran