Chemistry Reference
In-Depth Information
materials), centered on the goniometer of a diffractometer, and then exposed to the
X-rays. Therefore, cooling a single crystal means removing a very small amount of
heat from a very small sample (rotated in many positions during the experiment).
The air medium around the crystal is often a problem. The humidity of the air, in
fact, is enough to cause icing, whatever the method adopted to cool the sample.
The rapidity of currently available diffractometers equipped with area detectors
allows employing the so-called open flow systems, where a cryogenic fluid (He or
N
2
) is flushed onto the sample without recycling. It is clear that these methods could
be very expensive; however, liquid N
2
is typically available at low cost in many
laboratories. The most commonly adopted systems use liquid nitrogen both to
obtain an N
2
gas flow and to cool it to a temperature around the boiling point of
nitrogen (77 K) [
36
]. The temperature is then adjusted by electric warming of the
gas flow (in principle even above room temperature, guaranteeing uninterrupted
data collection over a very wide temperature range). Earlier systems regulated the
gas stream temperature by adjusting the evaporation of liquid nitrogen, a procedure
that however required large amount of N
2
(especially to reach lower temperatures).
In the last decade, open flow systems working with helium have been introduced,
despite the much higher costs of liquid or gaseous He. In experiments carried out at
Synchrotron work stations, the measurements are very rapid. This makes less prohibi-
tive the costs of experiments carried out with open flow He cryostats. Modern
equipment for X-ray diffraction in laboratories (using rotating anode generators,
multilayer optics, and area detectors) also allows rapid data collection. Therefore,
open flow He systems are not so uncommon in university laboratories. Helium gas
flow systems work using evaporation of the cryogenic medium [
37
,
38
], consumption
of which is proportional to the required temperature. These systems offer all the
advantages of open flow systems, namely optical access to the sample, rapid or even
flash cooling, low background of the gas stream. However, they also have some
inherent defects: large consumption of the cryogen (that makes them inadequate for
very long measurements), the necessity to have a warm outer stream to avoid
turbulence and ice formation. Some problems could be created by a thermocouple
in the gas stream, used to measure the flow temperature, because it could induce
turbulence and break the laminar flow, which is particularly fragile for a He stream.
A different solution is a He gas stream refrigerated through conduction by a two-
stage closed-cycle cooler, which works with compression/expansion cycles of He
gas. The advantage of the two-stage cooler is the higher temperature stability, but
the disadvantage is the base temperature (
10 K) and the temperature loss to reach
the sample (ca. 15 K). The smaller amount of cryogen used with this system makes
consumption very low, but the escaping attitude of low density He requires an open
cup at the end of the nozzle to bring the flow onto the sample. The cup could be
produced by a very light material (like beryllium) which is quite transparent to the
X-rays. Proper windows for incoming and outgoing primary beams will decrease
the scattering and therefore the background. An outer warm and dry stream is
necessary to prevent icing formation.
The two-stage (or even three-stage) closed cycle systems were actually invented
to cool the crystals directly through conduction, keeping the samples in closed
>
