High pressure measurements
If one considers the definition of pressure
p = F / A,
there are two possibilities to achieve large values of p:
- either the force F applied to the surface is large or
- the surface of area A upon which it acts is small.
In the first case, above a critical value of the force, the material of the pressure cell starts to deform and no further increase of pressure is possible. High pressures are therefore normally achieved by applying force on small surfaces.
Generally the force acts on two opposing anvils between which the sample is placed. The lateral flowing of the sample is prevented by a gasket acting as a container ring, and a pressure medium (ideally a liquid) ensures that the pressure is transmitted hydrostatically to the sample.
The Diamond Anvil Cell (DAC)
For the high pressure Mössbauer and NFS experiments a Diamond Anvil Cell (DAC) is used. Diamonds are used because they are among the hardest materials known and because their attenuation is low in the spectral range of x-rays. The following figure shows the principle design of the DAC used for the NRS high pressure experiments.
The diamonds are glued on a CuBe cylinder, centred on a hole that allows
The two CuBe plates on which the diamonds are glued are in turn inserted in larger CuBe plates, which are connected to each other by guiding pins and screws.
The guiding pins assure partly the constant parallelism of the diamonds as pressure is changed, while the screws are used to apply pressure.
The cell is quite compact: with a diameter of about 23 mm and a thickness of approximately 13 mm it can easily be used in a cryomagnet, where the space available for samples is generally limited.
Pressure determination
The pressure in the DAC can be determined from the pressure dependent shift of the fluorescence R line of ruby chips (Al2O3 doped with Cr3+). The fluorescence line appears as a doublet: at room temperature,- the R2-line has a wavelength
l2 = 692.70 nm and - the R1-line has a wavelength
l1 = 694.25 nm .
The wavelength of the R1-line is normally taken as a reference as this line is more intense than the R2. For pressures up to 30 GPa the pressure shift of the ruby R1 - line is linearly dependent on the pressure:
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To measure the fluorescence, the beam emitted by an Ar laser is focussed onto the pressure cell, in order to excite only one ruby chip at a time. The fluorescence radiation is collected and transported to a monochromator by an optical fibre. There it is analysed in energy, and the position of the R1-line can be determined. To have a precise determination of the pressure and of possible pressure gradients in the cell, the measurement is repeated on several different ruby chips, placed at different locations in the sample room.
As the system used for the pressure determination does not allow ïn situ pressure measurements, the pressure is always determined at room temperature before and after each measurement. Due to the different thermal expansion coefficients of the various parts of the cell, the pressure could increase when the temperature is lowered.