Pressure-transmitting Medium Research Articles

Compression of lithium fluoride to 92 GPa

The equation of state and compression behavior of lithium fluoride, LiF, have been determined to 92 GPa by X-ray diffraction in a diamond anvil cell. A neon pressure-transmitting medium was used to minimize the effect of differential stress on the sample. Consistent results using multiple pressure standards were obtained. By fitting the pressure–volume data to a Birch–Murnaghan equation of state, the isothermal bulk modulus and its corresponding pressure derivative at zero pressure were determined to be K0=70.1±0.7 GPa, and K′0=4.3±0.1. If the bulk modulus is fixed at 66.2 GPa, a value well constrained by independent elasticity measurements, we obtain K′0=4.6±0.1. The bulk modulus and its pressure derivative obtained from this work resolve the large discrepancy in previously reported values of K0 and K′0 for this material. Consequently, the equation of state of LiF is now sufficiently well constrained to allow its use as a pressure calibrant in high pressure experiments.

X-ray crystallography of simple molecular solids up to megabar pressures: application to solid oxygen and carbon dioxide

A review of recently developed methods to study the structural properties of simple molecular solids under pressures up to the megabar range using synchrotron X-ray diffraction techniques is presented. This includes the growth of single-crystals at high pressure and temperature conditions and the use of He as pressure-transmitting medium. The application of these methods to solid O2 and CO2 is then presented as illustrative examples, showing how they enabled to solve long-standing debates on the structure of the high-pressure crystalline phases of these compounds.

Role of the pressure transmitting medium on the pressure effects in DWCNTs

We present a high-pressure optical spectroscopy study on double-walled carbon nanotubes with 0.80 nm and 1.45 nm using nitrogen, argon, and alcohol-mixture as pressure transmitting medium (PTM). The pressure-induced redshift of the optical transitions in the outer tubes is very small below 10 GPa, demonstrating the enhanced mechanical stability due to the inner tube. An anomaly at the critical pressure 12 GPa signals the onset of the pressure-induced deformation of the tubular cross-sections. Using argon as PTM affects the results quantitatively: The value is lower and the absorption bands are broadened considerably. Furthermore, alcohol-mixture as PTM seems to destroy the tubes completely above 10 GPa due to its solidification. The results are compared with those for single-walled carbon nanotubes.

Linking dynamic to static pressure by laser interferometry

The primary calibration of pressure transducers is at present realized by static methods. This paper describes a new route to gaining traceability for dynamic calibration using the acousto-optic effect. The pressure range under consideration is up to 100 MPa. We set out a description of the general principle employed to gain traceability, the experimental set-ups that are used for the realization and the thermophysical background of the measurements, and some numerical estimates on the expected results for two different pressure-transmitting media are shown.

Anomalous compression and new high-pressure phases of vanadium sesquioxide, V2O3

We report results of a powder x-ray diffraction (XRD) study of vanadium sesquioxide, V2O3, under pressurization in a neon pressure-transmitting medium up to 57 GPa. We have established a bulk modulus value for corundum-type V2O3 of B0 = 150 GPa at B′ = 4. This bulk modulus value is the lowest among those known for the corundum-type-structured oxides, e.g. Al2O3, α-Fe2O3, Cr2O3, Ti2O3, and α-Ga2O3. We have proposed that this might be related to the difference in the electronic band structures: at room temperature V2O3 is metallic, but the above corundum-structured sesquioxides are semiconducting or insulating. Around ∼21–27 and ∼50 GPa we registered changes in the XRD patterns that might be addressed to phase transitions. These transitions were sluggish upon room-temperature compression, and hence we additionally facilitated them by the laser heating of one sample. We have refined the XRD patterns of only the first high-pressure phase in an orthorhombic lattice of a Rh2O3(II)-type. Our findings significantly extend the knowledge of the P–T phase diagram of V2O3 and advance the understanding of its properties. We speculate that the elastic properties of V2O3 can be closely linked to its electronic band structure and, consequently, we propose that slightly doped V2O3 (e.g. with Cr) could be a potential candidate for systems in which the bulk modulus value may be remarkably switched by moderate pressure or temperature.

Single-crystal X-ray diffraction at extreme conditions: a review

The latest developments in single-crystal X-ray diffraction at high pressure and high temperature are described. Advances in diamond anvil cell designs and X-ray sources allow collecting single-crystal diffraction data at pressures up and above 100 GPa and at temperatures above 1000°C. The technical details of single-crystal X-ray diffraction at high pressure such as the choice of pressure-transmitting media or the different methods for measuring pressures and temperatures have been reviewed. Examples of structural solution of complex structures and new materials, structural refinements of high pressure polymorphs as well as accurate compressibility data are described in order to outline the several advantages of using single crystals instead of powdered samples in high pressure diffraction experiments.

Raman spectroscopy of graphene at high pressure: Effects of the substrate and the pressure transmitting media

The pressure evolution of the Raman spectrum of graphene grown by chemical vapor deposition on polycrystalline copper is investigated with the use of polar and nonpolar pressure transmitting media (PTM). The $G$ and 2$D$ Raman bands exhibit similar pressure slopes for both PTM irrespective of any unintentional initial doping and/or strain of the samples. Our analysis suggests that any pressure-induced charge transfer effects are too small to influence the pressure response of graphene; rather, it is determined by the compressibility of the substrate and the graphene interaction with the substrate and the PTM. For the nonpolar PTM, a peculiar pressure behavior of graphene is observed in the PTM solidification regime, which resembles that of freestanding graphene.

Measurement of improved pressure dependence of superconducting transition temperature

We describe a technique for making electrical transport measurements in a diamond anvil cell at liquid helium temperature having in situ pressure measurement option, permitting accurate pressure determination at any low temperature during the resistance measurement scan. In general, for four-probe resistivity measurements on a polycrystalline sample, four fine gold wires are kept in contact with the sample with the help of the compression from the soft solid (usually alkali halides such as NaCl, KCl, etc.) acting as a pressure-transmitting medium. The actual pressure on the sample is underestimated if not measured from a ruby sphere placed adjacent to the sample and at that very low temperature. Here, we demonstrate the technique with a quasi-four-probe resistance measurement on an Fe-based superconductor in the temperature range 1.2–300 K and pressures up to 8 GPa to find an improved pressure dependence of the superconducting transition temperature.

Compression of scheelite-type SrMoO4 under quasi-hydrostatic conditions: Redefining the high-pressure structural sequence

The high-pressure behavior of tetragonal SrMoO4 was analyzed by Raman and optical-absorption measurements. Pressures up to 46.1 GPa were generated using diamond-anvil cells and Ne or N2 as quasi-hydrostatic pressure-transmitting media. A reversible phase transition is observed at 17.7 GPa. A second transition is found at 28.8 GPa and the onset of a third one at 44.2 GPa. The pressure dependence of Raman-active modes is reported for the different phases and the pressure evolution of the fundamental band-gap reported for the low-pressure phase. The observed changes in the Raman spectra contradict the structural sequence determined from previous experiments performed under higher non-hydrostaticity. This fact suggests that deviatoric stresses can influence pressure-driven transitions in scheelite-type oxides. We also report total-energy, lattice-dynamics, and band-structure calculations. They reproduce accurately the behavior of the physical properties of the low-pressure phase and predict the occurrence of phase transitions at pressures similar to experimental transition pressures. According to theory, the high-pressure phases have monoclinic and orthorhombic structures, which are much more compact than tetragonal scheelite. Theoretical results and experiments are compared with previous studies.

A 3 kbar hydrogen-compatible gas loader for Paris–Edinburgh presses

We present a device which allows compressed gases to be loaded into large volume opposed anvils used for high pressure neutron scattering in the multi-10 GPa range. The gases are initially loaded into clamps which can then be inserted into VX-Paris–Edinburgh load frames. The system is compatible with all inert gases as well as hydrogen and permits loading pressures of up to 3 kbar for which most gases have densities close to that of the liquid at ambient pressure. The device should have applications for the study of simple molecular solids as well as for loading gases as pressure-transmitting media.

How “Hollow” Are Hollow Nanoparticles?

Diamond anvil cell (DAC), synchrotron X-ray diffraction (XRD), and small-angle X-ray scattering (SAXS) techniques are used to probe the composition inside hollow γ-Fe(3)O(4) nanoparticles (NPs). SAXS experiments on 5.2, 13.3, and 13.8 nm hollow-shell γ-Fe(3)O(4) NPs, and 6 nm core/14.8 nm hollow-shell Au/Fe(3)O(4) NPs, reveal the significantly high (higher than solvent) electron density of the void inside the hollow shell. In high-pressure DAC experiments using Ne as pressure-transmitting medium, formation of nanocrystalline Ne inside hollow NPs is not detected by XRD, indicating that the oxide shell is impenetrable. Also, FTIR analysis on solutions of hollow-shell γ-Fe(3)O(4) NPs fragmented upon refluxing shows no evidence of organic molecules from the void inside, excluding the possibility that organic molecules get through the iron oxide shell during synthesis. High-pressure DAC experiments on Au/Fe(3)O(4) core/hollow-shell NPs show good transmittance of the external pressure to the gold core, indicating the presence of the pressure-transmitting medium in the gap between the core and the hollow shell. Overall, our data reveal the presence of most likely small fragments of iron and/or iron oxide in the void of the hollow NPs. The iron oxide shell seems to be non-porous and impenetrable by gases and liquids.

Formability of dual phase steels in electrohydraulic forming

Electrohydraulic forming (EHF) is based upon the electro-hydraulic effect: a complex phenomenon related to the discharge of high voltage electrical current through a liquid. In EHF, electrical energy is stored in a bank of capacitors and is converted into the kinetic energy needed to form sheet metal by rapidly discharging that energy across a pair of electrodes submerged in a fluid. The objective of this paper is to report the results of formability testing of dual phase steels in EHF conditions and to provide an explanation of the observed formability improvement based on analysis of the experimental results and through the use of a modeling technique developed as part of this work for simulating the EHF process. Comparison of the maximum strains resulting from EHF into a conical die and a v-shape die to the maximum strains resulting from quasistatic LDH testing (limiting dome height) indicated that substantially higher strains can be accomplished in the EHF process. In order to obtain the maximum benefits of increased formability with EHF technology, it is necessary to get to the high strain rate regime and the accompanying high hydrostatic stresses in the sheet, which occurs mostly in the contact area between the blank and the die. If the blank decelerates and gets to quasistatic forming conditions before filling the die cavity completely, very limited improvement in formability can be anticipated. A numerical model of the EHF process has been developed, incorporating four distinct models that are integrated into one: (1) an electrical model of the discharge channel, (2) a model of the plasma channel, (3) a model for the liquid as a pressure-transmitting medium, and (4) a deformable sheet metal blank in contact with a rigid die. The relative improvement in plane strain formability in EHF conditions was between 63% and 190%, depending on the grade of dual phase steel. Numerical modeling showed that the peak strain rates occurring in EHF are approximately 20,000 units per second.

Compressive behaviors of bcc bismuth up to 55 GPa

AbstractBismuth was compressed hydrostatically and nonhydrostatically up to 55 GPa in a diamond anvil cell. The strength of bcc bismuth was determined by the radial X‐ray diffraction (RXRD) technique: Y = −0.20(5) + 0.009(1)P, which is much smaller than that of normal pressure‐transmitting media (PTM), such as NaCl, Ar, and He. Different PTM [silicone oil (SO), Ar, and He] were used to determine the hydrostatical equation of state of bcc bismuth. The determined results are consistent with each other when different PTM were used, even when no PTM was used. The typical bulk modulus and its pressure derivation of bcc bismuth are 42.7(6) GPa and 5.3(1), respectively. The differential strain introduced by nonhydrostatic stress was derived from Hooke's law and lattice strain theory (LST). The differential strain in bismuth is very small even when no PTM was used. Because of the simple, stable structure and small strength, bcc bismuth is an ideal candidate for an internal pressure standard.

Effect of quasi-static prestrain on the formability of dual phase steels in electrohydraulic forming

Dual phase steels derive their name from their microstructure, which consists of islands of martensite surrounded by a ferrite matrix. These steels are increasingly being used in automobile structures in order to reduce weight, improve fuel economy, and maintain crash safety performance. The higher strength grades of dual phase steels, such as DP780 and DP980, often present significant formability challenges in sheet stamping operations, and therefore any technologies which could alleviate these issues would be of significant value to the automotive industry. Electrohydraulic forming (EHF) is based upon the electro-hydraulic effect: a complex phenomenon related to the discharge of high voltage electrical current through a liquid. In EHF, electrical energy is stored in a bank of capacitors and is converted into the kinetic energy needed to form sheet metal by rapidly discharging that energy across a pair of electrodes submerged in a fluid. During such a discharge, a high pressure, high temperature plasma channel is created between the tips of the electrodes. The resulting shockwave in the liquid, initiated by the expansion of the plasma channel, is propagated toward the blank at the acoustic velocity of the fluid, and the mass and momentum of the water in the shock wave accelerates the sheet metal blank toward the die. The objective of this paper is to report the results of formability testing of dual phase steels under three basic conditions: (1) conventional limiting dome height (LDH) testing; (2) starting with a flat blank and using one pulse of EHF to fill the desired die geometry; and (3) starting with a quasi-static preforming step to partially fill the die cavity and then using one pulse of EHF to fill the remaining area of the die cavity. A hybrid process which combines sheet hydroforming (HF) and EHF as described herein has the potential to reduce the cycle time of the EHF process by replacing the initial EHF forming increments with one quasi-static preforming step. Additionally, a numerical model was developed and employed in order to better understand the sheet deformation process within EHF. The numerical model consists of four distinct models that are integrated into one: (1) an electrical model of the discharge channel, (2) a model of the plasma, (3) a model of the liquid as a pressure-transmitting medium, and (4) a deformable sheet metal blank in contact with a rigid die. Significant improvements in formability were confirmed experimentally for DP780 and DP980 by forming into conical and v-shape dies using EHF from a flat sheet and by using EHF combined with a quasi-static preforming step. Numerical modeling showed that the peak strain rates occurring in both single-pulse EHF the hybrid HF-EHF process are approximately 17,000 units per second.

A compact device for loading diamond anvil cells with low-boiling pressure-transmitting media

A new compact laboratory device for loading diamond anvil cells with low-boiling pressure-transmitting media is described. This device allows, in particular, the easy loading of diamond anvil cells with pentane–isopentane (1:1) mixture at environmental temperatures up to 303 K and relative humidity at least up to 75–80%. The relatively simple design, the easy availability of materials used for the device and the application of liquid nitrogen as cooling agent allow the manufacture of such a device in a short time at almost any laboratory.

Cu-NQR of CeCu2Si2 under high pressure

We have measured 63Cu-NQR spectral lines and nuclear spin-lattice relaxation time T1 under pressure up to 5.4 GPa. Daphne oil 7474 is used as the pressure-transmitting medium to obtain good hydrostaticity. NQR frequency 63νq above 4 GPa suddenly decreases from the linear pressure dependence in the low pressure range (P ≤ 3.5 GPa). The observed sudden downward deviation of 63νq is associated with an increase of Ce valence. Above 4.5 GPa, the linear pressure dependence of 63νq is observed again, most likely related to the sharp crossover to the high valence state.

Piezochromic Luminescence Based on the Molecular Aggregation of 9,10‐Bis((E)‐2‐(pyrid‐2‐yl)vinyl)anthracene

Luminescent materials sensitive to environmental stimuli are of great interest from a scientific viewpoint owing to their potential applications in fluorescent switches and optical devices. Pressure is one of the most common natural external stimuli, and thus piezochromic materials, which show color changes resulting from external pressure or mechanical grinding, can be used as pressure-sensing and optical-recording systems. However, whereas pH-, light-, and temperature-sensitive materials are relatively well understood, studies of piezochromic materials remain inadequate owing to the absence of an effective mechanism to explain the relationship between changes in molecular assembly or packing and the corresponding luminescence properties of the material. Until now, successful systems have generally made use of transformations between monomeric and dimeric/excimeric states through hydrogen-bonding interactions as the mechanism to cause changes in luminescence. It is well-known that in the condensed phase, the luminescence properties of a given molecular system usually undergo significant variation according to the molecular aggregation state or stacking mode, since intermolecular interactions invariably alter photophysical processes. Therefore, an understanding of and the ability to control the molecular aggregation state and the consequent intermolecular interactions are still very important for the development of piezochromic materials. Herein we report an effective mechanism of piezochromic luminescence on the basis of the molecular aggregation state of 9,10-bis((E)-2-(pyrid-2-yl)vinyl)anthracene (BP2VA). BP2VA exhibited spectacular luminescence characteristics: grinding and the exertion of external pressure on the powder led to a change in its photoluminescence color from green to red. Three crystal polymorphs of BP2VA with different stacking modes involving gradually enhanced p–p interactions in the three crystalline states provided further insight into the origin of luminescence changes under the external stimulae. BP2VA was synthesized in a straightforward manner by a one-step Witting–Horner reaction according to a previously reported method, and the purified material was characterized by spectroscopic methods (see the Supporting Information). BP2VA powder exhibited a strong green emission at lmax= 528 nm, in contrast to its weak orange emission at lmax= 583 nm as a solution in THF. The weak orange emission was ascribed to a conformational relaxation in solution, which was reflected by the corresponding photoluminescence (PL) spectrum of BP2VA at low temperature (77 K; see Figure S1 in the Supporting Information). Furthermore, the emission of BP2VA aggregation as a solution in THF/water was blueshifted to lmax= 570 nm (from the value lmax= 583 nm in THF; see Figure S2). Interestingly, after being ground, BP2VA powder showed a strong red shift with a yellow emission (lmax= 561 nm) under UV light with a wavelength of 365 nm, and after being heated above 160 8C, the ground powder recovered its initial green emission (lmax= 528 nm; Figure 1b,c). The interconversion of the two states with their distinct emission colors is completely reversible through grinding and heating. The red shift of 33 nm in fluorescence emission upon grinding and the recovery of the initial state upon heating is a significant piezochromic effect. To gain further understanding of the piezochromic effect, we investigated the influence of applied pressure on the luminescence of BP2VA. The powder was placed in the holes (diameter: 200 mm) of a T301 steel gasket, which was preindented to a thickness of 50 mm. A small ruby chip was inserted into the sample compartment for in situ pressure calibration according to the R1 ruby fluorescence method. A 4:1 mixture of methanol and ethanol was used as a pressuretransmitting medium (PTM). The hydrostatic pressure on the powder was determined by monitoring the widths and separation of the R1 and R2 lines. The photoluminescence measurements under high pressure were performed on a QuantaMaster 40 spectrometer in the reflection mode. The 405 nm line of a violet diode laser with a spot size of 20 mm and a power of 100 mW was used as the excitation source. The diamond anvil cell (DAC) containing the sample was put on a Nikon fluorescence microscope to focus the laser on the sample. The emission spectra were recorded with a monochromator equipped with a photomultiplier. All experiments were conducted at room temperature. [*] Y. J. Dong, Dr. B. Xu, J. B. Zhang, L. J. Wang, J. L. Chen, Dr. H. G. Lv, Dr. S. P. Wen, Dr. B. Li, Dr. L. Ye, Prof. Dr. W. J. Tian State Key Laboratory of Supramolecular Structure and Materials Jilin Unversity Qianjin Street No. 2699, Changchun 130012 (China) E-mail: wjtian@jlu.edu.cn X. Tan, Prof. Dr. B. Zou State Key Laboratory of Superhard Materials Jilin University (China) E-mail: zoubo@jlu.edu.cn [] These authors contributed equally. [**] This research was supported by the 973 program (2009CB623605), the NSFC (Grant No. 20874035, 21074045, 21073071), and the Project of Jilin Province (Grant No.20100704). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201204660. . Angewandte Communications

Exploring the possible interlinked structures in single-wall carbon nanotubes under pressure by Raman spectroscopy

High-pressure Raman measurements on single-wall carbon nanotubes (SWNTs) have been carried out in a diamond anvil cell by using two wavelength lasers: 830 and 514.5 nm. Irrespective of using a pressure transmitting medium (PTM) or not, we found that nanotubes undergo similar transformations under pressure. The pressure-induced changes in Raman signals at around 2 and 5 GPa are attributed to the nanotube cross-section transitions from circle to ellipse and then to a flattened shape, respectively. Especially with pressure increasing up to 15–17 GPa, we observed that the third transition takes place in both the Raman wavenumber and the linewidth of G-band. We propose explanations that the interlinked configuration with sp3 bonds forms in the bundles of SWNTs under pressure, which was the cause for the occurrence of those Raman anomalies, similar to the structural-phase transition of graphite above 14 GPa. Our TEM observations and Raman measurements on the decompressed samples support this transition picture. Copyright © 2012 John Wiley & Sons, Ltd.

Complex high-pressure polymorphism of barium tungstate

We have studied BaWO${}_{4}$ under compression at room temperature by means of x-ray diffraction and Raman spectroscopy. When compressed with neon as a pressure-transmitting medium (quasihydrostatic conditions), we found that BaWO${}_{4}$ transforms from its low-pressure tetragonal structure into a much denser monoclinic structure. This result confirms our previous theoretical prediction based on ab initio calculations that the scheelite to BaWO${}_{4}$-II transition occurs at room temperature if kinetic barriers are suppressed by pressure. However, our experiment without any pressure-transmitting medium has resulted in a phase transition to a completely different structure, suggesting nonhydrostaticity may be responsible for previously reported rich polymorphism in BaWO${}_{4}$. The crystal structure of the low- and high-pressure phases from the quasihydrostatic experiments has been Rietveld refined. Additionally, for the tetragonal phase the effects of pressure on the unit-cell volume and lattice parameters are discussed. Finally, the pressure evolution of the Raman modes of different phases is reported and compared with previous studies.

High-resolution thermal expansion measurements under helium-gas pressure

We report on the realization of a capacitive dilatometer, designed for high-resolution measurements of length changes of a material for temperatures 1.4 K ≤ T ≤ 300 K and hydrostatic pressure P ≤ 250 MPa. Helium ((4)He) is used as a pressure-transmitting medium, ensuring hydrostatic-pressure conditions. Special emphasis has been given to guarantee, to a good approximation, constant-pressure conditions during temperature sweeps. The performance of the dilatometer is demonstrated by measurements of the coefficient of thermal expansion at pressures P ~/= 0.1 MPa (ambient pressure) and 104 MPa on a single crystal of azurite, Cu(3)(CO(3))(2)(OH)(2), a quasi-one-dimensional spin S = 1/2 Heisenberg antiferromagnet. The results indicate a strong effect of pressure on the magnetic interactions in this system.