Pressure-transmitting Medium Research Articles

Structural stability and compressive behavior of ZrH2under hydrostatic pressure and nonhydrostatic pressure

The important transition metal dihydride ZrH2 has been characterized using in situ synchrotron X-ray diffraction combined with diamond anvil cell techniques and ab initio calculations. The effect of a pressure-transmitting medium on the structural stability and compressive behavior was investigated under both hydrostatic pressure and nonhydrostatic pressure conditions. The ambient I4/mmm structure is stable under both nonhydrostatic and hydrostatic compressions. The supplementary theoretical calculations have proposed that the I4/mmm structure transformed into the P4/nmm structure above 100 GPa confirming the stability of the I4/mmm structure during the experimental runs. The difference in the volume reduction between the two compressions becomes larger with increasing pressure. Up to about 50 GPa, the volume collapse of nonhydrostatic compression is 6% relative to the hydrostatic compression. The present study offers a new approach with nonhydrostatic compression or shear stress for finding higher volumetric density hydrogen structures in metal hydrides.

Integrated-fin gasket for palm cubic-anvil high pressure apparatus.

We described an integrated-fin gasket technique for the palm cubic-anvil apparatus specialized for the high-pressure and low-temperature measurements. By using such a gasket made from the semi-sintered MgO ceramics and the tungsten-carbide anvils of 2.5 mm square top, we successfully generate pressures over 16 GPa at both room and cryogenic temperatures down to 0.5 K. We observed a pressure self-increment for this specific configuration and further characterized the thermally induced pressure variation by monitoring the antiferromagnetic transition temperature of chromium up to 12 GPa. In addition to enlarge the pressure capacity, such a modified gasket also improves greatly the surviving rate of electrical leads hanging the sample inside a Teflon capsule filled with the liquid pressure-transmitting medium. These improvements should be attributed to the reduced extrusion of gasket materials during the initial compression.

High-pressure Raman scattering of CaWO₄ up to 46.3 GPa: evidence of a new high-pressure phase.

The high-pressure behavior of CaWO4 was analyzed at room temperature by Raman spectroscopy. Pressure was generated using a diamond-anvil cell and Ne as pressure-transmitting medium. The pressure range of previous studies has been extended from 23.4 to 46.3 GPa. The experiments reveal the existence of two reversible phase transitions. The first one occurs from the tetragonal scheelite structure to the monoclinic fergusonite structure and is observed at 10 GPa. The onset of a previously unknown second transition is found at 33.4 GPa. The two high-pressure phases coexist up to 39.4 GPa. The Raman spectra measured for the low-pressure phase and the first high-pressure phase are consistent with previous studies in the pressure range where comparison is possible. The pressure dependence of all the Raman-active modes is reported for different phases. We also report total-energy and lattice-dynamics calculations, which determine the occurrence of two phase transitions in the pressure range covered by the experiments. The first transition is in full agreement with experiments (scheelite-to-fergusonite). According to calculations, the second-highest pressure phase has an orthorhombic structure (space group Cmca). Details of this structure, its Raman modes, and its electronic band structure are given. The reliability of the reported results is supported by the consistency between the theoretical and experimental values obtained for transition pressures, phonon frequencies, and phonon pressure coefficients.

HP-induced supra-molecular organization of guest molecules in FER-type zeolites

The response to pressure of a synthetic all-silica ferrierite (Si-FER) and of a natural ferrierite from Monastir (Sardinia, Italy) (Mon-FER, Na0.56 K1.19 Mg2.02 Ca0.52 Sr0.14)(Al6.89Si29.04)O72 ·17.86 H2O) is here investigated combining HP synchrotron XRPD experiments and molecular dynamics simulations. The experiments were carried out by using penetrating (methanol:ethanol:water 16:3:1, m.e.w.; ethanol:water 1:3, e.w.) and non-penetrating (silicone oil, s.o.) pressure transmitting media (PTM). In Si-FER compressed in e.w., both water (w.) and ethanol molecules (e.) enter the pore system even at 0.2 GPa. The structural refinement of the data collected at 0.8 GPa reveals 8 w. and 4 e. molecules in the 10- and 6-membered ring channels, in tight agreement with the results of MD simulations. In Si-FER compressed at 0.2 GPa in m.e.w., only water molecules penetrate the 10-membered ring channels (15 per u.c.), organized in chains running along the channel axis. The interactions among the guest species and the framework oxygen atoms are very weak, due to the hydrophobicity of the framework. Upon decompression, the intruded extra-molecules are not completely released, so giving rise to new materials with different extra-framework contents. The results obtained for Si-FER compressed in m.e.w. and s.o. were compared to those obtained for Mon-FER, demonstrating that the zeolite composition and the PTM strongly influence the overall elastic parameters of the investigated samples. Specifically, Mon-FER shows a much higher rigidy than Si-FER in both media, due to the stiffening effect of the numerous extraframework species present in the natural sample. The higher rigidity of Si-FER in m.e.w. with respect to s.o. can be explained by the penetration, in the former case, of the PTM molecules, which contribute to stiffen the framework.

HP-induced penetration of guest molecules in high-Si mordenite

A high-Si mordenite (HS-MOR, SiO2/Al2O3 ~ 200, s.g. Cmcm) was investigated by in-situ synchrotron XRPD under HP using silicone oil (s.o.) as non-penetrating pressure transmitting medium (PTM), and the following penetrating PTM: (16:3:1) methanol: ethanol:water (m.e.w), (3:1) water:ethanol (w:e), ethylene glycol (e.g.) and resorcinol (res). The experiments were performed in DAC at SNBL1 (ESRF, Grenoble). The evolution of the structural features was followed by full profile Rietveld refinements. In the Pamb-1.2 GPa range, the volume contraction of HS-MOR compressed in s.o. (Tab. 1) is the highest found among the HS zeolites studied up to now in the same P range [1-2]. Above this P value, a rapid and irreversible loss of long range order is observed in the diffraction patterns. These findings suggest a gradual P-induced amorphization. The main results of the experiments with penetrating PTM are the following (Tab. 1 and Fig. 1): i) no complete X-ray amorphization and phase transitions achieved up to the highest investigated P; ii) penetration of additional guest species into the channels, even at very low P; iii) lower cell-volume reduction with respect to that found in s.o. in the same P range; iv) partial reversibility of the P-induced effects upon decompression. The lower compressibility of HS-MOR in penetrating PTM with respect to s.o. is due to the entrapping of additional guest molecules, which contributes to sustaining the mordenite framework and stiffening the material.

Tuning Polymorphs using Pressure-Transmitting Media

The role of pressure-transmitting media is to ensure that uniaxial pressure is translated into a hydrostatic pressure. Many of these media are useful to high-pressure scientists for a limited pressure regimen out with which the media becomes non-hydrostatic in nature. For most pressure studies the role of the media is purely to apply the pressure however in recent years the media has been used to dissolve compounds of interest before precipitating these out by the application of pressure. Previous work of Fabbiani et al gave a wonderful example of how changing the concentration of the solution and hence the pressure of precipitation can isolate new polymorphs of the pharmaceutical material, piracetam.[1] It is known that the structural changes that occur in a material may depend on the pressure that is applied i.e. phase transition may not occur under hydrostatic regime whereas they will if put under non-hydrostatic environments. Our present studies have been exploring the role of pressure in the polymerisation reaction of simple systems and structurally characterising the materials preceding these events.[2] These studies have provided extra structural insight into the previous Raman studies.[3] We present here a case study where the role of the pressure-transmitting medium extends beyond just application of pressure but where, depending on the medium chosen, new phases can be observed. This work has been conducted at the Pearl beamline at ISIS Neutron Facility in UK.

Instrumentation development for crystallography at high-pressure

Historically high-pressure (HP) research has been an area that is heavily dependent on the availability of the experimental equipment. Many of the discoveries in HP science followed promptly from breakthroughs in instrumentation development, which provided researchers with higher pressure limits or larger sample volumes. A limited availability of commercial pressure cells and the need to remain at the cutting edge of the research make it likely that anyone working in this field will at some point engage in designing new or in modifying existing HP equipment. This presentation aims to introduce an engineering approach to developing pressure cells and to present such generic tools as computer aided design (CAD) and finite element analysis (FEA). The use of engineering methods in the design of HP equipment will be illustrated using recently developed pressure cells. This includes some new devices for neutron scattering such as gas-driven sapphire anvil pressure cell for changing pressure at cryogenic temperatures in neutron diffraction experiments [1]. Another example is a gas loader for the P-E press which can be used to load gases into the sample space at elevated pressures for subsequent studies of gases and gas mixtures as well as for use of gases as pressure-transmitting media to pressures of over 18 GPa [2]. The examples of use of FEA for miniaturization of the pressure cells and their components will include miniature pressure cells for X-ray diffraction with cryo-flow refrigerators shown in the Figure below [3].

Preparation of carbon nanotube monoliths by high-pressure compaction

High-pressure compaction was used to produce monolithic multiwall carbon nanotubes (MWCNTs) from different sources: (1) high-purity commercial Baytubes®, (2) chemical-vapor deposited MWCNTs without purification at the Laboratory of Production of CNT/UNIFRA, and (3) the same MWCNTs as (2) purified with HCl. Pressures of 4.0 GPa and 7.7 GPa were applied at room temperature using two different pressure-transmitting media, lead and graphite. Cylindrical monolithic MWCNTs with diameters of about 6 mm were obtained. The samples were characterized by Raman spectroscopy, X-ray diffraction, elemental analysis, N2 adsorption and transmission electron microscopy. Results showed that the best sample was obtained with MWCNTs without purification, containing residues of MgO catalyst, and using lead as the pressure-transmitting medium at 7.7 GPa. High-pressure may cause compressive stress and shear stress for the MWCNTs. The lead container, as a quasi-hydrostatic pressure-transmitting medium, provided more compressive stress than shear stress while the impurities acted as binding materials. Both helped to obtain better densification of the MWCNTs. [New Carbon Materials 2014, 29(3): 193–202]

<span class="aps-inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>α</mi><mo>−</mo><mi>ε</mi></math></span> transition pathway of iron under quasihydrostatic pressure conditions

We have carefully investigated the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\epsilon}$ transition pathway of iron under quasihydrostatic pressures. For this purpose, combined measurements of extended x-ray absorption fine structure (EXAFS) and x-ray magnetic circular dichroism at the Fe $K$ edge were performed using a helium pressure-transmitting medium. Collapse of the ferromagnetism simultaneously occurs with the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\epsilon}$ structural transition, which is in contrast to the scenario that the transition is driven by the pressure-induced instability of the ferromagnetism in $\ensuremath{\alpha}$ phase of iron. We conclude that shear stress is important to initiate the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\epsilon}$ transition. Our model to fit the EXAFS profile demonstrates that the local atomic arrangement in $\ensuremath{\epsilon}$ phase is slightly distorted due to unfinished shuffle motion, whereas shear motion finishes at the beginning of the transition.

Quasi-hydrostatic X-ray powder diffraction study of the low- and high-pressure phases of CaWO4 up to 28 GPa

We have studied CaWO4 under compression using Ne as pressure-transmitting medium at room temperature by means of synchrotron X-ray powder diffraction. We have found that CaWO4 beyond 8.8 GPa transforms from its low-pressure tetragonal structure (scheelite) into a monoclinic structure (fergusonite). The high-pressure phase remains stable up to 28 GPa and the low-pressure phase is totally recovered after full decompression. The pressure dependence of the unit-cell parameters, as well as the pressure–volume equation of state, has been determined for both phases. Compared with previous studies, we found in our quasi-hydrostatic experiments a different behavior for the unit-cell parameters of the fergusonite phase and a different transition pressure. These facts suggest that deviatoric stresses influence on the high-pressure structural behavior of CaWO4 as previously found in related compounds. The reported experiments also provide information on the pressure dependence of interatomic bond distances, shedding light on the transition mechanisms.

Preparation of carbon nanotube monoliths by high-pressure compaction

High-pressure compaction was used to produce monolithic multiwall carbon nanotubes (MWCNTs) from different sources: (1) high-purity commercial Baytubes®, (2) chemical-vapor deposited MWCNTs without purification at the Laboratory of Production of CNT/UNIFRA, and (3) the same MWCNTs as (2) purified with HCl. Pressures of 4.0 GPa and 7.7 GPa were applied at room temperature using two different pressure-transmitting media, lead and graphite. Cylindrical monolithic MWCNTs with diameters of about 6 mm were obtained. The samples were characterized by Raman spectroscopy, X-ray diffraction, elemental analysis, N2 adsorption and transmission electron microscopy. Results showed that the best sample was obtained with MWCNTs without purification, containing residues of MgO catalyst, and using lead as the pressure-transmitting medium at 7.7 GPa. High-pressure may cause compressive stress and shear stress for the MWCNTs. The lead container, as a quasi-hydrostatic pressure-transmitting medium, provided more compressive stress than shear stress while the impurities acted as binding materials. Both helped to obtain better densification of the MWCNTs.

In-situ high-pressure powder X-ray diffraction study of α-zirconium phosphate.

The high-pressure structural chemistry of α-zirconium phosphate, α-Zr(HPO4)2·H2O, was studied using in-situ high-pressure diffraction and synchrotron radiation. The layered phosphate was studied under both hydrostatic and non-hydrostatic conditions and Rietveld refinement carried out on the resulting diffraction patterns. It was found that under hydrostatic conditions no uptake of additional water molecules from the pressure-transmitting medium occurred, contrary to what had previously been observed with some zeolite materials and a layered titanium phosphate. Under hydrostatic conditions the sample remained crystalline up to 10 GPa, but under non-hydrostatic conditions the sample amorphized between 7.3 and 9.5 GPa. The calculated bulk modulus, K0 = 15.2 GPa, showed the material to be very compressible with the weak linkages in the structure of the type Zr-O-P.

High-pressure-induced structural changes, amorphization and molecule penetration in MFI microporous materials: a review.

This is a comparative study on the high-pressure behavior of microporous materials with an MFI framework type (i.e. natural mutinaite, ZSM-5 and the all-silica phase silicalite-1), based on in-situ experiments in which penetrating and non-penetrating pressure-transmitting media were used. Different pressure-induced phenomena and deformation mechanisms (e.g. pressure-induced over-hydration, pressure-induced amorphization) are discussed. The influence of framework and extra-framework composition and of the presence of silanol defects on the response to the high pressure of MFI-type zeolites is discussed.

Quasi-hydrostatic Limit of LiF as a Pressure Transmitting Medium and Its Equation of States

Quasihydrostatic limit of LiF as a pressure transmitting medium is investigated by synchrotron radiation x-ray diffraction combined with the diamond anvil cells technique up to 60GPa at room temperature. The equation-of-state parameters of LiF are determined to be V0 = 65.7(2) Å3, B0 = 58(3) GPa and B′0 = 4.9(2) in the silicon oil environment; V0 = 67.4(3) Å3, B0 = 51(3) GPa and B′0 = 4.7(2) without pressure transmitting medium. The full width at half maximum of LiF (111) peak increases with the increase of pressure in two independent experiments. The pressure distribution in the sample chamber is estimated by line-scanning x-ray diffraction measurements across the chamber's center, which presents as homogeneous with Pmax − Pmin of about 1GPa below 40GPa.

Pressure-induced phase transitions in coesite

High-pressure behavior of coesite was studied on single crystals using diamond-anvil cells with neon as the pressure-transmitting medium by means of in situ Raman spectroscopy up to pressures of ~ 51 GPa. The experimental observations were complemented with theoretical computations of the Raman spectra under similar pressure conditions. We find that coesite undergoes two phase transitions and does not become amorphous at least up to ~ 51 GPa. The first phase transition (coesite I to coesite II) is reversible and occurs around 23 GPa. The second transition (coesite II to coesite III) at about 35 GPa is also reversible but involves a large hysteresis. Samples recovered from the highest pressure achieved, ~ 51 GPa, show Raman spectra of the initial coesite. The ab initio calculations gave insight into the initiation mechanism of the first phase transition, implying, from the analysis of unstable phonon modes, that it is probably a displacive phase transition due to shearing of the four-membered rings of SiO4 tetrahedra upon compression. The transition to the lowest-symmetry phase, coesite III, is possibly a first-order phase transition that leads to a very distinct structure. None of the metastable high-pressure phases of coesite has been previously studied and it was widely accepted that coesite undergoes pressure-induced amorphization at significantly lower pressures (30 GPa). The study of the high-pressure behavior of coesite is important to better constrain the metastable phase diagram of silica. Further crystallographic investigations are necessary for characterizing the structures of these metastable coesite forms. Crystalline or amorphous metastable phases derived from coesite under high-pressure conditions are of particular interest because they can be used as potential tracers of peak transient pressures (stress) reached in processes such as impacts or faulting.

Unusual compression behavior of nanocrystalline CeO₂.

The x-ray diffraction study of 12 nm CeO2 was carried out up to ~40 GPa using an angle dispersive synchrotron-radiation in a diamond-anvil cell with different pressure transmitting medium (PTM) (4:1 methanol: ethanol mixture, silicone oil and none) at room temperature. While the cubic fluorite-type structure CeO2 was retained to the highest pressure, there is progressive broadening and intensity reduction of the reflections with increasing pressure. At pressures above 12 GPa, an unusual change in the compression curve was detected in all experiments. Significantly, apparent negative volume compressibility was observed at P = 18–27 GPa with silicone oil as PTM, however it was not detected in other circumstances. The expansion of the unit cell volume of cubic CeO2 was about 1% at pressures of 15–27 GPa. To explain this abnormal phenomenon, a dual structure model (hard amorphous shell and relatively soft crystalline core) has been proposed.

Pressure transmitting medium-dependent structure stability of nanoanatase TiO2 under high pressure

The pressure transmitting medium (PTM)-dependent structure stability of ∼12 nm nanoanatase TiO2 under high pressure has been investigated using in situ synchrotron X-ray powder diffraction techniques at room temperature. We found that the starting anatase phase (I41/amd) transforms into the monoclinic baddeleyite phase (P21/c) regardless of embedded in PTM or not. However, the beginning and ending pressure for the transformation under quasihydrostatic compression is found to be ∼7 GPa higher than that of under nonhydrostatic compression, and amorphization of sample occurs more easily during the former condition. This result indicates that the interface energy between the PTM and nanoparticles undoubtedly modifies the phase stability during high pressure experiments.

Pressure-induced water intrusion in FER-type zeolites and the influence of extraframework species on structural deformations

The response to pressure of a natural ferrierite from Monastir (Sardinia, Italy) (Mon-FER) and of the synthetic all-silica phase (Si-FER) was investigated by means of in situ synchrotron X-ray powder diffraction in the presence of penetrating (methanol:ethanol:water 16:3:1, m.e.w.) and non-penetrating (silicone oil, s.o.) pressure transmitting media (PTM). The following aspects are discussed: (i) the penetration of extra-H2O molecules into the all-silica phase and the related structural aspects involving both framework and extraframework systems; (ii) the influence of the zeolite composition and of the different PTM used in the compression experiments on the overall elastic parameters of the two investigated samples; (iii) the elastic parameters and the unit cell P-induced deformations of the mineral phase; (iv) the reversibility extents of the observed phenomena. Evidence of H2O molecules penetration during compression of Si-FER in m.e.w. was found. The refinement performed at 0.2 GPa enabled location of 15 H2O molecules forming bulk-like and monodimensional H2O clusters. No methanol or ethanol penetration was observed. The results demonstrate that the free volume of the porous material is not the only parameter influencing water condensation since applied pressure is an additional fundamental factor. The bulk modulus value calculated from Pamb to 4.9 GPa for Mon-FER (K0=44 GPa, Kp=0.2) is intermediate between the lowest (about 14 GPa) and the highest (about 72 GPa) values determined to date for zeolites compressed in non-penetrating PTM. The highest P-induced deformations are observed for Si-FER compressed in s.o. In general, higher rigidity for the natural sample was found in both media.

Effect of H2O on the Pressure-Induced Amorphization of AlPO4-54·xH2O

Microporous AlPO4-54·xH2O, which exhibits the largest pores among zeolites and aluminophosphates with a diameter of 12.7 Å, was investigated at high pressure by X-ray powder diffraction and Raman spectroscopy in diamond anvil cells. The material was found to begin to amorphize near 2 GPa using either a nonpenetrating pressure transmitting medium (PTM) silicone oil or no PTM. When H2O is used as a PTM, amorphization begins at a lower pressure of 0.9 GPa. In this case, superhydration effects are observed and higher relative unit cell volumes are observed prior to the beginning of pressure-induced amorphization (PIA) as compared to the experiment in silicone oil due to insertion of the H2O molecules in the pores. In all cases, in these experiments at room temperature, amorphization was irreversible. Ex situ experiments were used to investigate the local structure of pressure-amorphized AlPO4-54·xH2O by nuclear magnetic resonance and by X-ray absorption spectroscopy, which show that, upon increasing pressure, two water molecules enter in the coordination sphere of IVAl, thereby increasing the coordination number from 4 to 6, which destabilizes the structure. The present results show that the insertion of and/or reaction with guest species can be used to strongly modify the stability of microporous materials with respect to PIA.

Room-temperature vibrational properties of multiferroic MnWO4 under quasi-hydrostatic compression up to 39 GPa

The multiferroic manganese tungstate (MnWO4) has been studied by high-pressure Raman spectroscopy at room temperature under quasi-hydrostatic conditions up to 39.3 GPa. The low-pressure wolframite phase undergoes a phase transition at 25.7 GPa, a pressure around 8 GPa higher than that found in previous works, which used less hydrostatic pressure-transmitting media. The pressure dependence of the Raman active modes of both the low- and high-pressure phases is reported and discussed comparing with the results available in the literature for MnWO4 and related wolframites. A gradual pressure-induced phase transition from the low- to the high-pressure phase is suggested on the basis of the linear intensity decrease of the Raman mode with the lowest frequency up to the end of the phase transition.