Diamond anvil cell optical measurements

Huffman, D. R., L. A. Schwalbe, and D. Schiferl, 1982. Use of smoke samples in diamond-anvil cells to measure pressure dependence of optical spectra application to the ZnO exciton, Solid State Commun. (in press). 

In the next section the rare-earth compounds that have been studied by optical means under pressure so far will be reviewed. Then, after a brief introduction of the most commonly used high pressure device, the diamond anvil cell, sect. 4 presents a discussion of the pressure-induced changes of the crystal-field levels and their interpretation. In sects. 5 and 6 some aspects of the dynamical effects under pressure are discussed. These include lifetime and intensity measurements, the influence due to excited configurations and charge transfer bands, and the electron-phonon coupling. 

Meanwhile, the diamond anvil cell (DAC) has become the dominant device in high-pressure research. Although optical studies build up a big part of research performed with DACs, nearly every method used to study physical properties of matter has been successfully employed also in high-pressure DACs. Examples are electrical measurements (Gonzalez et al., 1986), X-ray diffraction (Hazen and Finger, 1982 Holzapfel, 1989), X-ray absorption (Tolentino et al., 1990), Mossbauer spectroscopy (Lubbers et al., 2000), neutron scattering (Vettier, 1989), resonance techniques (Sakai and Pifer, 1985). 

Thin films of silicates have been produced by pressing powders in a diamond anvil cell (Hofmeister 1997), by cutting grain samples to submicron thick slices with an ultra-microtome (Bradley et al 1999), by electron-beam evaporation (Djouadi et al. 2005), and by laser deposition in a vacuum (Brucato et al. 2004). On one hand, powders produced in a laboratory are directly measured in transmittance when they are embedded in a matrix of transparent materials (e.g. KBr or polyethylene). On the other hand, reflectance measurements do not require the use of matrices powders of selected-size grains are directly measured with an appropriate optical accessory. Through measurements in both transmittance and reflectance, it is possible to evaluate the optical constants of a material. These are certainly the physical parameters... 

Experiments have established that there are four polymorphs in solid iron (a-, y-, S-, and s-phases). Saxena et al (1993) proposed a fifth iron phase (/3-phase) based on changes in thermal emission while laser heating the sample in a diamond-anvil cell. Boehler (1993) also observed similar changes in optical properties of iron in the same P-T range. Subsequent in situ X-ray diffraction measurements in the laser-heated diamond-anvil cell supported the occurrence of this new iron phase, although the structure of this phase is still under debate (Saxena et al, 1996 Yoo et al, 1996 Andrault et al, 1997, 2000 Saxena and Dubrovinsky, 2000). However, this phase was not observed in... 

Barnett JD, Block S, Piermarini GJ (1973) An optical fluorescence system for quantitative pressure measurement in the diamond-anvil cell. Rev Sci Instrum 44 1-9 Batlogg B, Maines RG, Greenblatt M, DiGregorio S (1984) Novel p-V relationship in ReOs under pressure. Phys Rev B 29 3762-3764... 

The optical absorption of small samples subjected to a hydrostatic pressure is usually measured in a diamond anvil cell (DAC). There are several types of DACs, differing mainly in the way in which the pressure is transmitted to the cell [17]. Some of these cells, like the so-called Merril-Basset one, have been modified for absorption spectroscopy at low temperature [12]. The basic part of a DAC is shown in Fig. 4.9. It is made of two diamonds separated by an... 

A diamond-anvil-cell (DAG) is a small high pressure cell most suitable for the spectroscopic measurement of molecular or atomic diffusion. The DAG is used for various kinds of spectroscopic investigations on liquids and solids at pressures up to several tens of GPa [19-22]. The optically transparent nature of diamond over a wide wavelength span allows in situ optical measurements in combination with conventional equipment such as visible light or infrared spectrometers. The protonic diffusion in ice is measured by a traditional diffusion-couple method, in the present case, with an H2O/D2O ice bilayer. The mutual diffusion of hydrogen (H) and deuteron (D) in the ice bUayer is monitored by measuring the infrared vibrational spectra. The experimental details are described in the following sections. 

Figure 24.5 Schematic drawing of the optical configuration for measuring the infrared reflection spectra (left) a diamond anvil cell containing an ice bilayer and (right) a reflection objective for focusing incident infrared lights. (From Ref [24]).

Large-scale diamond compression (high-pressure) cells have been available for some time. However, during recent years, a microversion of the cell, also known as the diamond anvil cell, has been produced. This small diamond anvil cell is ideal for the study of physically hard (but compliant) samples, certain intractable samples, and samples that are optically too thick for normal transmission measurements. Note that this small form of the accessory is intended for samples 1 mm or less in size. The aperture through the diamonds is small, and therefore the use of a beam condenser accessory is recommended for optimal throughput. Alternatively, the cell may be used in combination with an IR microscope. 

Barnett, J. D. Block, S. and Piermarini, G. J. (1973) "An optical fluorescence system for quantitative pressure measurement in the diamond-anvil cell". Rev. Sci. Instrum. 44, 1-9. 

To reach pressures above 15 GPa the B4C anvils have to be replaced by a stronger material. The diamond anvil cell has been proven most suitable during the last decade. Unfortunately, due to the high costs of the anvils the dimensions of the sample have to be kept small typically, the area of the sample is about a factor of 100 smaller as compared to B4C anvils. For X-ray and optical measurements this is not a problem. For Mossbauer experiments diamond anvil cells have been employed only with transitions which exhibit a large recoil-free fraction, in particular with Fe, and the lanthanide Eu. The spectrometer to be described is based on a design reported earlier (Bassett et al. 1967, Huber et al. 1977). Experiments can be carried out at cryogenic temperatures. 

The opposed anvil cell consists of two optical anvils and a gasket, located between the parallel faces of the two opposing anvils. Samples are placed in the hole of the gasket and are pressurized when the opposed anvils are pushed towards each other. The most common material for anvils is diamond. For mid and far infrared spectra, type Ila diamonds are used, while low-fluorescent type la diamonds are used for Raman spectroscopic measurements [5]. We have also devised a glass anvil cell for Raman spectroscopic measurements [6], and a calcium fluoride anvil cell for infrared spectroscopic measurements [7] with attainable working pressures of 13 and 6 kbar, respectively. Diagrams, for the interested reader, of the window and opposed anvil cells can be found in reference 1. 

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... 

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