Calibration, diamond anvil cell

Fig. 1 Schematic diagram of a diamond anvil cell (DAC). The sample, pressure calibrant and hydrostatic fluid are loaded into the gasket hole, which is then compressed between the culets of the two diamond anvils...

Infrared and Raman studies at very high pressure (up to several hundred kbar) are carried out fairly routinely with diamond anvil cells (DAC). The DAC, which was first developed for high-pressure infrared absorption measurements by Weir et al. (1959) and for X-ray studies by Jamieson et al. (1959), has become a very powerful tool for a wide variety of ultra-high pressure investigations, with particularly important applications in solid state physics. The potential of the method has increased enormously with the introduction of gaskets into the DAC by Van Valkenburg (see Jayaraman, 1983) and with the possibility of pressure calibration by the ruby fluore.scence method (Forman et al, 1972). 

In situ pressure measurements within the diamond anvil cell are performed by means of calibration substances that are embedded in the pressure chamber together with the sample and the transmitting medium. Frequently used... 

Figure 11. Raman spectra of the formic-biological system (A) shown with the vibration peaks of formic and diamond anvils used in this study. The outlined boxed region is shown at higher resolution (B) to quantify the successive decrease in the peak intensity of the C-H stretch of formic acid at pressures of 68,142, and 324 MPa. The equivalent formate concentrations (C), corresponding to each peak height change, are based on comparisons with a known calibration curve. All experiments were performed at 25°C, with diamond anvil cells with gold-lined sample chambers. Pressures were estimated using Raman shifts in quartz used as an internal calibrant.

Recently, a high-pressure investigation of CaSi2 in a diamond anvil cell has been published. Nitrogen was used as a pressure-transmitting material and the diamond anvil cell was calibrated by the ruby luminescence method. 

The determination of the pressure inside the diamond anvil cell requires a calibrated standard. The most commonly used standard is the R-line ( E —> Aj) emission from ruby. The wavelengths of the ruby Rj and Rj lines have been accurately calibrated as a function of pressure using fixed point standards [86] and the measured lattice constant of NaCl in conjunction with the Decker equation of state [87-89]. The shift rate of the lower energy Rj line is normally used to calibrate pressure. The room temperature Ri shift rate is linear up to 200 kbar (0.365 A/kbar (- 0.759 cm Vkbar)) [87]. Above 200 kbar, the shift becomes non-linear and has been quantified empirically [90-93]. The currently accepted Rj line wavelength calibration is valid up to 800 kbar and can be expressed [91] by... 

Fig. 10.1 Diamond anvil cell (left, viewed down the load axis) and its schematic right, adapted with permission from [16]) 1 anvil (diamond), 2 backing plate (beryUium), 3 gasket (e.g. tungsten), 4 sample chamber with pressure medium (liquid or gas), calibration standard (ruby) and the crystal. Note the smallness of the working volume...

In this section, the mechanism of the piezoelectricity presented imaginary in section 5 will be confirmed under applied pressure. Pressures applied to the single crystals using a diamond anvil cell (DAC) as shown in Figure 20 were calibrated by the ruby fluorescence technique. The DAC was putted on the four-circle diffractmeter. The unit cell parameters were refined by using the 2d-a) step scan technique. 

The diamond anvil cell (DAC) is one of the most common instruments used for achieving hydrostatic pressures in the 5-150-kbar (0.5-15-GPa) range [20]. The cell is characterized by a very small sample volume, typically a cylinder of 100 /uim in diameter and 50 fxm high, located between the parallel facets of two diamonds, as shown in Fig. 21 the diamonds are typically about 2 mm thick. The cell is defined by a metal gasket, usually made of stainless steel, that forms the seal between the two diamonds. The sample is placed in the cell with the pressure medium, usually a type of oil or organic solvent, and a small piece of ruby. The pressure dependence of the wavelength of the ruby fluorescence at approximately 695 nm is usually used for the pressure calibration [21]. 

The 5P4E is a very good Raman scatterer, whose Raman spectrum is dominated by an intense band at 1001 cm related to the trigonal mode of vibration of the aromatic rings within the 5P4E molecules. The pressure sensitivity of this mode has been calibrated in diamond anvil cells by several authors [22-24]. The value of K=3.3 cm /GPa given by Laplant et al. [24] was used in the present study. The pressure resolution of our measurements is about 30 MPa for this lubricant and the Raman spectrometer used. 

Raman studies can be carried out at high pressure, using a diamond-anvil cell [40]. To calibrate the pressure, a small piece of powdered ruby can be enclosed in a metal gasket. Using the 514.5-nm line of an Ar laser, the sharp ruby Ri fluorescence line can be excited. This line exhibits a pressure-dependent shift (—0.753 cm /kbar) [41], The diamond-anvil cell is used in the 180° backscattering geometry. 

Figure 3-1 Diamond anvil high-pressure cell, (a) Detail of diamond cell, (b) Side view, (c) Front view. A and B, parts of piston C, hardened steel insert D, presser plate E, lever G, screw H, calibrated spring. (Reproduced with permission from Ref. 1.)...

In a diamond cell, the sample volume is sacrificed for the sake uf higher pressures, and hence, all operations connected with (he cell have lo be performed under a microscope. In preparing the DAC for an experiment, the first step is to indent Ihe metal gasket (hardened stainless steel strip nr Inconel strip) with the anvil diamonds to the correct thickness (5(1 to UK) micrometers) and then drill a IOO- to 200-micrumeter hole as close lo the center of the indentation as possible. The gasket is seated on the face of one of the diamonds in the same orientation as it had when the indentation was made. The sample material and a small chip of ruby for pressure calibration are (lien placed in the hole. Finally, to maintain hydrostatic pressure the hole is tilled with a tiny drop of fluid from a syringe and then (he hole is quickly sealed by (he diamond fucus before Ihe fluid evaporates. 

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