Sample In Diamond Anvil Cell Research Articles

Crystal-structure properties and the molecular nature of hydrostatically compressed realgar

The structure of realgar, As4S4, and its evolution with pressure have been investigated employing in situ X-ray diffraction, optical absorption and vibrational spectroscopy on single-crystal samples in diamond-anvil cells. Compression under true hydrostatic conditions up to 5.40 GPa reveals equation-of-state parameters of V 0 = 799.4(2.4) A3 and K 0 = 10.5(0.4) GPa with $$K_0^\prime$$ = 8.7. The remarkably high compressibility can be attributed to a denser packing of the As4S4 molecules with shortening of the intermolecular bonds of up to 12 %, while the As4S4 molecules remain intact showing rigid-unit behaviour. From ambient pressure to 4.5 GPa, Raman spectra exhibit a strong blue shift of the Raman bands of the lattice-phonon regime of 24 cm–1, whereas frequencies from intramolecular As-S stretching modes show negligible or no shifts at all. On pressurisation, realgar shows a continuous and reversible colour change from bright orange over deep red to black. Optical absorption spectroscopy shows a shift of the absorption edge from 2.30 to 1.81 eV up to 4.5 GPa, and DFT calculations show a corresponding reduction in the band gap. Synchrotron-based measurements on polycrystalline samples up to 45.5 GPa are indexed according to the monoclinic structure of realgar.

High-pressure behaviours of HoMn2O5 and BiMn2O5

High-pressure behaviours of isomorphousHoMn2O5 andBiMn2O5 (Pbam, Z = 4) were studied, using Raman spectroscopy and single-crystal x-ray diffraction,with the samples in diamond anvil cells pressurized to 10.09 GPa and 8.54 GPa,respectively, at room temperature. No phase transition was detected ineither material. The two manganates are most compressible along thea axis with the latticecontractions along the b and c directions nearlythe same. Their P–V data fitted with the second-order Birch–Murnaghan equation of state (B′ = 4.0) give thebulk moduli B0 = 138(2) GPa for BiMn2O5 and B0 = 173(3) GPa for HoMn2O5. The difference in bulk moduli results fromBiMn2O5 being morecompressible than HoMn2O5 along the a axis. Our study shows that the Mn–O frameworks in isomorphousRMn2O5 compounds(R = Bi and rare earth) are not rigid at high pressures, but rather adapt to the behaviour of thecoordination spheres around the R atoms. Bond valence calculations indicate that thepressure-induced charge transfers between the two Mn sites have different characters for thetwo compounds.

Nanoprobe measurements of materials at megabar pressures

The use of nanoscale x-ray probes overcomes several key limitations in the study of materials up to multimegabar (> 200) pressures, namely, the spatial resolution of measurements of multiple samples, stress gradients, and crystal domains in micron to submicron size samples in diamond-anvil cells. Mixtures of Fe, Pt, and W were studied up to 282 GPa with 250-600 nm size synchrotron x-ray absorption and diffraction probes. The probes readily resolve signals from individual materials, between sample and gasket, and peak pressures, in contrast to the 5-microm-sized x-ray beams that are now becoming routine. The use of nanoscale x-ray beams also enables single-crystal x-ray diffraction studies in nominally polycrystalline samples at ultrahigh pressures, as demonstrated in measurements of (Mg,Fe)SiO(3) postperovskite. These capabilities have potential for driving a push toward higher maximum pressures and further miniaturization of high-pressure devices, in the process advancing studies at extreme conditions.

Electrical resistance of Sn<sub>2</sub>P<sub>2</sub>S<sub>6</sub> crystals under pressure in diamond anvil cell

Measurements of the electrical resistance versus pressure up to 3.5 GPa and room temperature are realized for Sn 2 P 2 S 6 samples in diamond anvil cell. It is established, that electrial resistance decreases with increasing pressure. Equations describing pressure dependences R(p), α T (p), E gR (T,p) of Sn 2 P 2 S 6 are presented. Parameters of presented equations are obtained by approximation method. Discontinuities of pressure dependences of the electrical resistance are observed, which are possibly related to a new structural phase transition uder pressure in Sn 2 P 2 S 6 at p≈3GPa .

The advanced ion-milling method for preparation of thin film using ion slicer: Application to a sample recovered from diamond-anvil cell

The advanced argon ion-milling technique using a new instrument called ion slicer was newly developed for preparation of thin foil. Compared to the conventional ion-milling methods, this technique facilitates very wide area to be homogeneously thinned by rocking the ion beam source with low angle and the specimen during milling. Here we applied this technique to a sample recovered from a laser-heated diamond-anvil cell (DAC). We obtained the thin film of almost entire cross section of the DAC sample along the compression axis, which possesses of approximately (10x50) microm(2). The laser-heated sample is often heterogeneous due to a large temperature gradient. However the chemical analyses are obtained from the whole hot spot under the transmission electron microscope by preparing the thin foil using the ion slicer.

A scanning angle energy-dispersive X-ray diffraction technique for high-pressure structure studies in diamond anvil cells

A scanning angle diffraction technique with an energy-dispersive solid-state detector (SSD) and white synchrotron radiation has been developed for high-pressure structure studies in diamond anvil cells (DACs). This technique is similar to the CAESAR technique [Y. Wang, T. Uchida, R. Von Dreele, M.L. Rivers, N. Nishiyama, K. Funakoshi, A. Nozawa, and H. Kaneko, A new technique for angle-dispersive powder diffraction using an energy-dispersive setup and synchrotron radiation, J. Appl. Cryst. 37 (2004), p. 947] developed for large-volume presses, but extended to DAC applications with high spatial resolution. The main feature of the technique is the well-defined collimation in the beam path to the detector, which improves the signal-to-noise ratio significantly, compared to routine monochromatic angle-dispersive powder diffraction with area detectors. This is particularly useful and essential for low-scattering materials and for amorphous and liquid diffraction/scattering studies using DACs. Data collected from crystalline and amorphous samples in DACs show that a coarse 2θ scan (0.1–0.2° for crystals and 0.5° for amorphous structure) is sufficient to obtain reasonable diffraction resolution. The scanning angle energy-dispersive X-ray diffraction technique provides angle-dispersive X-ray diffraction (ADXD) data in multiple energies. Such multi-energy ADXD data carry much more information than regular single-energy ADXD, which could provide site-specific atomic structure information for full structure refinement.

Stability of (Mg0.5Fe0.5)O and (Mg0.8Fe0.2)O magnesiowüstites in the lower mantle

We have heated magnesiowustites (Mg 0.5 Fe 0.5 )O and (Mg 0.8 Fe 0.2 )O to temperatures of over 1000 K at pressures of over 80 GPa simulating the stability of the solid solution at physical conditions relevant to the Earth9s lower mantle. The X-ray study of the externally heated samples in diamond-anvil cells shows that magnesiowustite may dissociate into a magnesium-rich and an iron-rich oxide component. The result is important because the decomposition of magnesiowustite into lighter and heavier phases will cause dynamic effects that could lead to mantle heterogeneity.

Measurement of temperature distributions across laser heated samples by multispectral imaging radiometry

Two-dimensional temperature mapping of laser heated diamond anvil cell samples is performed by processing a set of four simultaneous images of the sample, each obtained at a narrow spectral range in the visible to near infrared. The images are correlated spatially, and each set of four points is fitted to the Planck radiation function to determine the temperature and the emissivity of the sample, using the gray body approximation. The method is tested by measuring the melting point of Pt at 1 bar and measuring laser heated Fe at 20 GPa in the diamond anvil cell. The accuracy and precision are shown to compare well to standard spectroradiometry, and the effect of imaging resolution on the measured distribution is evaluated. The principal advantages of the method are (1) the temperature and emissivity of the sample are mapped in two dimensions; (2) chromatic aberrations are practically eliminated by independent focusing of each spectral band; and (3) all of the spectral images are obtained simultaneously, allowing temporal variations to be studied. This method of measuring temperature distributions can be generalized to other hot objects besides laser heated spots.

Thickness measurement of sample in diamond anvil cell

We report on an original method that measures sample thickness in a diamond anvil cell under high pressures. The method is based on two hypotheses: completely plastic deformation on the gasket and completely elastic deformation of the diamonds. This method can further eliminate the effect of diamond deformation on the thickness measurement of a sample, which permits us to measure the thickness of alumina up to 41.4 GPa.

Finite element analysis of resistivity measurement with four point probe in a diamond anvil cell

Using finite element analysis, we studied the steady current field distribution under the configuration of four point probe method for resistivity measurement in a diamond anvil cell (DAC). Based on the theoretical analysis, we made a correction to the formula by Valdes [L. B. Valdes, Proc IRE, 42, 420 (1958)]. The results show that our formula provides more accurate determination of sample resistivity, especially when the sample thickness is less than the probe spacing. We found that finite size of the electrode could lead to significant errors in resistivity measurement for semiconducting samples. We also found that the probe spacing is a key factor in the resistivity measurement accuracy for samples in DAC. When the sample thickness t is close to the probe spacing s, the error becomes larger and reaches a maximum.

Ab initio 0 K isotherm for crystalline 1,1-diamino-2,2-dinitroethylene

The ab initio calculation of the 0 K isotherm of the organic molecular crystal 1,1-diamino-2,2-dinitroethylene (C2H4N4O4), also known as FOX-7, is accomplished by means of solutions of the many-body Schrödinger equation in a periodic crystal lattice. It was found that the Hartree–Fock method is adequate to represent the behavior of the material and that, in general, density functional methods give inferior results. Initially, calculations were done assuming rigid molecules under compression. In further calculations the internal molecular bond lengths were optimized for each value of compression. Finally, calculations were performed in which all the molecular coordinates were optimized. The results are compared with experimental x-ray diffraction data obtained by compressing samples in diamond anvil cells. Excellent agreement is obtained when the molecular coordinates are completely optimized. In addition, FOX-7 is highly anisotropic and this anisotropy must be taken into account to obtain good agreement with hydrostatic compression data.

INFRARED SPECTRA OF CHLOROFORM AT HIGH PRESSURES

ABSTRACT Infrared spectra of chloroform samples in diamond anvil cells at ambient temperature and at pressures up to 10 GPa are reported. The freezing pressure is determined to be 0.76 GPa. All six fundamental vibrational modes and several combination modes are observed, with some exhibiting several components as a result of crystal field effects, and the dependence of their peak wavenumbers on pressure is presented. Some minor changes in the spectra near 6 GPa may be indicative of a structural phase transition, postulated from previous Raman studies. No color changes are observed over this pressure range and molecular distortion appears to be minimal, although there is evidence of a moderate increase in Cl-C-Cl bending forces. A metastable crystal is formed when the liquid is superpressed and flash-frozen, and its spectra suggest that this is characterized by molecular orientational disorder, similar to that found in crystalline bromoform.

RAMAN AND INFRARED SPECTRA OF IODOFORM AT HIGH PRESSURES

ABSTRACT Raman and infrared spectra are reported for iodoform samples in diamond anvil cells at ambient temperature and at pressures up to 5 GPa (Raman) and 10 GPa (infrared). The spectra appear to evolve smoothly and no evidence of any structural phase transitions is found. The dependence on pressure of 7 Raman and 13 infrared peak wavenumbers is presented. A large increase in intermolecular bonding strengths is confirmed, together with a moderate increase in intramolecular I-C-I bending forces. Color changes in the samples at high pressures are found to be mostly reversible, but long exposure to high fluxes of visible photons causes some molecular dissociation, with the release of iodine.

INFRARED SPECTRA OF BROMOFORM AT HIGH PRESSURES

Infrared spectra in the wavenumber ranges 100–1800 and 2700–4800 cm−1 are reported for bromoform samples in diamond anvil cells at ambient temperature and at pressures up to 10 GPa. The freezing pressure is estimated to be 0.13 ± 0.02 GPa. The spectra appear to evolve smoothly and no major discontinuities are detected. The dependence on pressure of eleven peak wavenumbers (five fundamentals and six combinations) is presented. All modes show small percentage increases in wavenumber over this pressure range, except for the degenerate bend, ν6, which exhibits a 20% increase, suggesting that the equilibrium Br-C-Br angles may be slightly changing with increasing pressure.

Synchrotron radiation and laser heating in a diamond anvil cell

The last decade has seen the development of very bright third generation x-ray synchrotron sources that have significantly improved x-ray diffraction experiments at high pressure and high temperature. In the same time, focusing optics as well as detectors have been improved in such a way that x-ray diffraction experiments can be routinely carried out on samples in laser-heated diamond anvil-cell, i.e., under extreme static pressure and temperature conditions. It is now possible to obtain from a laser heated sample in a diamond anvil cell (DAC) very high quality powder patterns, needed for accurate in situ phase boundary and pressure-volume-temperature equation of state determinations, as well as for structural refinements. The setup for in situ x-ray investigation at simultaneous high pressure and temperature is complex, since perfect alignment of x-ray path, infrared-laser hot spot, and optical temperature measurement is required. It provides, however, the most accurate information on the structural behavior with step by step increases of pressure and temperature. In addition, a range of experiments can also be realized at room temperature after laser annealing of DAC samples, which has been shown very efficient in releasing stresses build on compression, and/or overcoming phase transitions kinetic barriers.

Pressure–volume–temperature paths in the laser-heated diamond anvil cell

The temperature, pressure, and stress conditions in the diamond anvil cell sample chamber before, during, and after laser heating are mapped by employing standard materials as in situ pressure markers. Unit cell volumes of Pt, MgO, and NaCl were monitored by synchrotron-based x-ray diffraction at temperatures between 300 and 2290 K and pressures ranging from 14 to 53 GPa. To aid in interpreting the resulting pressure–volume–temperature paths, we perform a series of model calculations of the high-temperature, high-pressure x-ray diffraction behavior of platinum subjected to a general stress state. Thermal pressure and thermal expansion effects within the laser-heated volume are observed but are not sufficient to fully explain the measured paths. Large apparent pressure changes can also result from relaxation of deviatoric stresses during heating and partial reintroduction of those stresses during quench. Deviatoric stresses, monitored from both diffraction peak widths and lattice parameter shifts as a function of (hkl), may significantly distort equation of state results if it is assumed that the sample is under hydrostatic pressure. Large-scale, nearly isothermal pressure relaxation events are observed at ∼2000 K. It is proposed that these arise from relaxation of heated components (pressure medium, gasket, cell itself) outside of the directly laser-heated volume.

Infrared Spectra of Benzene at High Pressures

Mid-infrared spectra in the ranges 400-1800 and 2700-4000 cm-1 are reported for benzene samples in diamond anvil cells at ambient temperature and pressures up to 10 GPa. The freezing pressure is confirmed to be 0.1±0.05 GPa. Changes in the spectra and in the sample appearance indicate that sluggish solid state phase transitions occur near 2.0 and 4.0 GPa. The wave number shifts with pressure for 27 selected peaks are plotted. Their small increases show that only minor distortions of the molecules occur, and there is no evidence of any weakening of intramolecular bonds in this pressure range.

Infrared Spectra of Liquid and Crystalline Ethanol at High Pressures

Mid-infrared spectra in the ranges 400–1800 and 2700–4600 cm−1 of ethanol samples in diamond anvil cells at ambient temperature and pressures up to 11 GPa are reported. The freezing pressure is confirmed to be 1.8 GPa, and, unlike methanol, the resulting solid is crystalline rather than glassy. No further phase transitions are observed in this pressure range. The wave number shifts of 30 selected peaks with pressure are deduced, and their small magnitudes indicate that only minor distortions of the molecules occur. The effects of the strengthening of the intermolecular hydrogen bonds with pressure on the internal modes are briefly discussed.

Infrared Spectra of Crystalline, Glassy and Liquid Methanol at High Pressures

Mid-infrared spectra in the range 400–1800 cm−1 of methanol samples in diamond anvil cells at ambient temperature and pressures up to 11 GPa are reported. The freezing pressure is confirmed to be 3.6 GPa, and the spectra of the resulting metastable glass are very similar to those of the liquid. When maintained at high pressure, the glass spontaneously transforms to an ordered crystalline phase which is stable over the range 3.6 to 11 GPa. Small changes in peak wavenumbers for 14 internal modes as a function of pressure are observed, indicating that distortion of the molecules is minimal. A slight decrease for the C-O-H bending mode is attributed to charge transfer from the molecular 0-H bond to the strengthening intermolecular hydrogen bond.

Far-infrared spectra of solid methanol at high pressures

Far-infrared spectra of methanol samples in diamond anvil cells at ambient temperature and at pressures up to 11 GPa are reported. The liquid forms a metastable glass at about 3.6 GPa, the absorption spectrum of which consists of a single broad band which shifts to higher wavenumbers with increase of pressure. On maintaining the pressure at a high value, the glass usually transforms to a crystal and at least five distinct peaks are then observed. The evolution of these peak wavenumbers with pressure indicates that no further transitions occur. An increase in hydrogen bond strength by a factor of about 1.5 is deduced for the crystal over the pressure range 3.6 to 11 GPa.