Pressure calibration

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 

Other fluorescent pressure sensors have also been suggested, but are used less frequently than ruby. Wavelength calibrations with pressure have been reported for alexandrite [97,98],Sm + Y3Al50i2 (YAG) [99-101],Sm + MFCl (M = Ba,Sr) [102,103],Sm2+ SrB407 [104,105],Eu + YAG [106],Tm + YAG [107],Nd + YAIO3 [108], and V + MgO [109]. Fluorescence lifetime calibrations have also been re- 

Calibrations based on techniques other than fluorescence are also available. Lattice parameter calibrations from pressure dependent X-ray experiments have been reported for several systems. The most commonly used X-ray standards include MCI (M=Na, K, Cs) [89], Au [117,118], W [119], and Cu [120]. Spectral shifts of Raman lines in N2 [121] and diamond [122] have also been calibrated as a function of pressure. 

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Decker, D.L., Bassett, W.A., Merrill, L., Hall, H.T., and Barnett, J.D., High-Pressure Calibration-Critical Review, J. Phys. Chem. Ref. Data 1 (3), 773-835... 

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...

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. 

Fig. 2. Basic principle of the diamond cell. Pressure is generated in the gasket hole when the diamonds are pushed against one another. The sample and a small chip of ruby for pressure calibration are placed in the hole and the latter is filled with a pressure-transmitting medium. The purpose of the gasket is to provide containment for the pressure medium as well as support the diamond Dais. Suitable apertures in the diamond support Mocks provide access to optical, x-ray. and other radiation...

A further requirement for a pressure sensor is the stability of the host lattice at high pressures and temperatures. This requirement strongly narrows the range of possible candidates for high-pressure sensors. From X-ray diffraction experiments under pressure, it was found that YAG is stable at least up to 69 GPa at room temperature (Liu and Vohra, 1993). This is one of the reasons why doped YAG was chosen by many researchers as a promising host for pressure calibrants. 

Density compensation may also be accomplished through electronic circuitry. Some systems compensate for density changes automatically through the design of the level detection circuitry. Other applications compensate for density by manually adjusting inputs to the circuit as the pressurizer cools down and depressurizes, or during heatup and pressurization. Calibration charts are also available to correct indications for changes in reference leg temperature. 

Pressure calibration is necessary in pressure work, and this is accomplished by incorporating a small ruby crystal with the sample under study. The Ruby scale (4) was developed by the National Bureau of Standards (now the NIST) in 1972, and the sharp Ruby Ri fluorescent line has been calibrated vs. pressure by NIST, and is suitable even up to megabar pressures (5). 

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). 

The pressure generated in a sample chamber is first calibrated against the supplied load in reference to the pressure fixed points listed in Table 5. This pressure calibration is carried out at room temperature by detecting changes in the electric resistance of these standard materials as they transform to their denser phases. For accuracy of temperature and pressure at elevated temperatures, an additional cahbration is done utilizing melting of gold, silver, copper, and various solid-state transitions. 

Schmitt R. T. (2000) Shock experiments with the H6 chondrite Kernouve pressure calibration of microscopic shock effects. Meteorit. Planet. Sci. 35, 545—560. 

Longhi J. (2003) Green glasses new pressure calibration, new ascent mechanism, new calculations, same story. In Lunar Planet. Sci. XXXIV, 1528. The Lunar and Planetary Institute, Houston (CD-ROM). 

Bohlen S. R. (1984) Equilibria for precise pressure calibration and a frictionless furnace assembly for the piston-cyUnder apparatus. N. Jb. Mineral. Mh. 9, 404-412. 

Gupta YM, Shen XA (1991) Potential use of the raby R2 hne shift for static high-pressure calibration. Appl Phys Letts 58 583-585... 

Another method for pressure calibration is to add a powdered salt... 

The pressure calibration factor k takes into account losses of species caused for example by the intensity distribution of the molecular beam from the Knudsen cell or the transmission of the ion source and the analyzer. Three different methods are generally used [86] ... 

Method 3 is an absolute method for pressure calibration in contrast to the relative methods 1 and 2. A variant of method 3 is possible by the use of a vacuum microbalance (see e.g. Hiipert [51] and Biefeld [78]). 

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