Pressure-generation with diamond anvils

All static studies at pressures beyond 25 GPa are done with diamond-anvil cells conceived independently by Jamieson [32] and by Weir etal [33]. In these variants of Bridgman s design, the anvils are single-crystal gem-quality diamonds, the hardest known material, truncated with small flat faces (culets) usually less than 0.5 nun in diameter. Diamond anvils with 50 pm diameter or smaller culets can generate pressures to about 500 GPa, the highest static laboratory pressures equivalent to the pressure at the centre of the Earth. 

The principle of pressure-generation with opposed diamond anvils is shown in fig. 1. A gasket with a central hole is placed between the two diamond tips to provide a sample chamber which can be filled with a pressure-transmitting medium to maintain hydrostatic conditions. Furthermore, the sample chamber is loaded with the sample and some pressure sensor. 

Simple compression cells with diamond windows have appeared recently. Although they generate lower pressures than traditional diamond anvil cells, they can provide excellent spectra from a range of samples that are too thick for direct measurements. 

Pressures and temperatures equivalent to that at the base of the upper mantle and top of the lower mantle can be achieved with multi-anvil devices. Hall (1958) described a tetrahedral multianvil device used to generate pressures high enough to synthesize diamond. Further developments led to the octahedral multi-anvil device (Kawai and Endo, 1970), refined by many Japanese scientists (e.g., Akaogi and Akimoto,1977 Onodera, 1987 Ohtani, 1987). Walker et al. 

Pressure was generated with a diamond anvil cell (DAC) employing beveled anvils with central flats ranging from 20 to 100 jim and flat diamonds with 200-500 pm culets. Two types of DAC were used modified (to match a continuous flow He cryostat) Mao-Bell cell for operations at room and low temperatures [41] and a Mao-Bell high-T external heating cell [42]. The latter one is equipped with two heaters and thermocouples. Four experiments were performed at RT aiming to highest pressure and the final pressures varied from 180 to 268 GPa. For low-temperature measurements we used a continuous-flow He cryostat, which allowed infrared and in situ Raman/ fluorescence measurements. More details about our IR/Raman/fluorescence setup at the NSLS are published elsewhere [41]. 

To be referred to next is the most modern diamond anvil type, generating pressures of order of 10-100 GPa. The cell illustrated in Figure 6(e) consists of two gem diamonds with optically flat surfaces, between which a sample confined in a drilled hole of a thin metal gasket is sandwiched. To attain isostatic compression an inert gas or an organic liquid, like a 4 1 volume mixture of methanol and ethanol, is contained with the sample. The generated pressure is measured directly from the pressure shift of the fluorescence line of ruby powder mixed with the sample. Temperatures to 5000 K can be obtained by laser heating. The quantity of sample confined in a typically 0.1-mm-wide hole is extremely small, just a few microcrystals. At present, research has focused on in situ observations using X-ray and other optical methods, rather... 

Fig. 1. Schematic depiction of a diamond anvil cell. The upper half shows an enlargement of the opposed diamond anvils that are used to generate pressure. The sample is placed in the central hole of the metal gasket along with a pressure transmitting fluid and a pressure cali-brant. The lower half of the figure illustrates one method for applying mechanical force to the diamonds. The resulting translational force leads to a reduction in sample volume and a consequent increase in sample pressure...

The maximum pressure that can be generated by DAC is 5000 kbar, whereas only 100 kbar can be generated with SAC [27, 28]. Sapphire as well as diamond anvils should be selected with respect to transparency and lack of luminescence and... 

A study of RDX behaviour under static ultrahigh pressure (up to 65 GPa), generated by using diamond anvil cells (DACs) using FT-IR spectroscopy and UV-VIS absorption spectroscopy, is a topic of paper [61]. RDX changed its colour into dark red when compressed up to 20 GPa with caesium iodide (Csl), filled as a pressure medium. In this case the intensities of characteristics of IR absorption peaks of RDX decreased as the pressure increased, and did not return to the intensities measured at ambient pressure, after the pressure was unloaded. However, when RDX was compressed alone, its colour changed into yellow at a pressure above 60 GPa. In its UV-VIS absorption spectra the peak shifted from 243.5 nm at ambient pressure to 410 nm at 65.5 GPa. The authors assume that the HOMO-LUMO band gap of this nitramine decreases with increasing pressure [61] which is in line with the opinion of Kuklja and Kunz [59]. 

Fig. 1. Principle of the pressure-generating method with opposed diamond anvils. The enlargement on the right side shows the central hole of the gasket filled with rubies, a sample, and a pressure-transmitting medium.

---