High-pressure Form Research Articles

Structure of pressure-induced amorphous form ofSnI4at high pressure

The structure of pressure-induced amorphous form of ${\text{SnI}}_{4}$ has been studied by synchrotron x-ray diffraction measurements using diamond-anvil cell. The structure factor, the reduced radial distribution function, and the density were obtained between 25 and 55 GPa. Comparison of measured structure factor with that calculated for a randomly oriented dimerized molecules model shows no similarity between them. Experimentally obtained radial distribution function provides clear evidence for the absence of both distances between tin and iodine atoms and between iodine atoms within the ${\text{SnI}}_{4}$ tetrahedral molecule. Characteristic features of the structure factor for amorphous ${\text{SnI}}_{4}$ show a marked resemblance to those for elemental metallic glasses which are successfully reproduced by a model of dense random packing of equal-sized spheres. Indeed, various quantities such as positions of peaks in measured structure factors and in experimental radial distribution functions, the coordination number are in precise agreement with those derived from the dense random-packing model. We conclude that the structure of the high-pressure amorphous form of ${\text{SnI}}_{4}$ is the relatively rigid packing of tetrahedra composed of iodine and substitutional tin atoms.

Ionic high-pressure form of elemental boron

Ionic high-pressure form of elemental boron

The preparation and structure of salty ice VII under pressure

It is widely accepted that ice, no matter what phase, is unable to incorporate large amounts of salt into its structure. This conclusion is based on the observation that on freezing of salt water, ice expels the salt almost entirely as brine. Here, we show that this behaviour is not an intrinsic physico-chemical property of ice phases. We demonstrate by neutron diffraction that substantial amounts of dissolved LiCl can be built homogeneously into the ice VII structure if it is produced by recrystallization of its glassy (amorphous) state under pressure. Such 'alloyed' ice VII has significantly different structural properties compared with pure ice VII, such as an 8% larger unit cell volume, 5 times larger displacement factors, an absence of a transition to an ordered ice VIII structure and plasticity. Our study suggests that there could be a whole new class of 'salty' high-pressure ice forms.

Ionic high-pressure form of elemental boron

Boron is an element of fascinating chemical complexity. Controversies have shrouded this element since its discovery was announced in 1808: the new 'element' turned out to be a compound containing less than 60-70% of boron, and it was not until 1909 that 99% pure boron was obtained. And although we now know of at least 16 polymorphs, the stable phase of boron is not yet experimentally established even at ambient conditions. Boron's complexities arise from frustration: situated between metals and insulators in the periodic table, boron has only three valence electrons, which would favour metallicity, but they are sufficiently localized that insulating states emerge. However, this subtle balance between metallic and insulating states is easily shifted by pressure, temperature and impurities. Here we report the results of high-pressure experiments and ab initio evolutionary crystal structure predictions that explore the structural stability of boron under pressure and, strikingly, reveal a partially ionic high-pressure boron phase. This new phase is stable between 19 and 89 GPa, can be quenched to ambient conditions, and has a hitherto unknown structure (space group Pnnm, 28 atoms in the unit cell) consisting of icosahedral B(12) clusters and B(2) pairs in a NaCl-type arrangement. We find that the ionicity of the phase affects its electronic bandgap, infrared adsorption and dielectric constants, and that it arises from the different electronic properties of the B(2) pairs and B(12) clusters and the resultant charge transfer between them.

Methane-water clusters under pressure: Are clathrate cages optimal clusters?

Global cluster structure optimization is applied to CH(4)(H(2)O)(n), n=4-21, in a simple model but with the additional application of external pressure. The combined effect of pressure and of the presence of a methane molecule leads to a structural transition away from the well-known pure water cluster structures. The exact location of this transition is a function of pressure and of cluster size, but it invariably leads to "nestlike" structures as high-pressure form. Clathratelike cages around the methane molecule in the cluster center are less favorable than the best nests for all sizes and at all pressures studied, but they do become low-energy local minima at high pressures.

H2-O2 supercritical combustion modeling using a CFD code

The characteristics of propellant injection, mixing, and combustion have a profound effect on liquid rocket engine performance. The necessity of raising rocket engines performance requires a combustion chamber operation often in a supercritical regime. A supercritical combustion model based on a one-phase multi-components approach is developed and tested on a non-premixed H2-O2 flame configuration. A two equations turbulence model is used for describing the jet dynamics where a limited Pope correction is added to account for the oxidant spreading rate. Transport properties of the mixture are calculated using extended high pressure forms of the mixing rules. An equilibrium chemistry scheme is adopted in this combustion case, with both algebraic and stochastic expressions for the chemistry/turbulence coupling. The model was incorporated into a computational fluid dynamics commercial code (Fluent 6.2.16). The validity of the present model was investigated by comparing predictions of temperature, species mass fractions, recirculation zones and visible flame length to the experimental data measured on the Mascotte test rig. The results were confronted also with advanced code simulations. It appears that the agreement between the results was fairly good in the chamber regions situated downstream the near injection zone.

POST-PEROVSKITE: THE LAST MANTLE PHASE TRANSITION.: K. Hirose, J. Brodholt, T. Lay, D. Yuen, Eds. (2007) American Geophyscial Union, Geophysical Monograph Series, 174. 350 pages, hardbound. ISBN 13: 978-0-87590-439-9. $104.00 (AGU Member Price: $72.80).

We live in interesting times. It used to be that mantle phase transitions were so few in number that even a child could become an expert—but my childhood is long past. Now even seismologists invoke obscure phase transitions to elicit favorable publishing decisions whenever an unexpected wiggle arises in a seismogram, and even mineral physicists routinely discuss seismic anisotropy. (Note to self: good reason not to drop optical mineralogy from a syllabus.) This surprising broadening of knowledge arose from the singular alliance of large-scale facilities provided by synchrotron light sources, supercomputers, and seismological data facilities like IRIS. Kei Hirose, John Brodholt, Thorne Lay, and David Yuen marshal the forces of this effort in their collection of papers, Post-perovskite, The Last Mantle Phase Transition, which highlights the knowns and unknowns of the Pbnm to Cmcm transition in MgSiO3 perovskite (pv) to post-perovskite (ppv). One known is that the seismic features in D″ are largely attributable to it. Oh, D″? That’s the bottom of the Earth’s mantle, a place where Birch’s famous warning applies: “Language undergoes a transformation to high pressure forms.” Now reread the prepenultimate sentence. Possible hype notwithstanding, the editors provide a nice disciplinary packaging of the papers: experimental and theoretical mineral physics, seismology, and dynamics. The experimental section begins with two histories of high-pressure experimentation, a more general one by Yagi on mantle phase transitions, and a more ppv-centric one by Hirose. Their thrill of discovery is evident. While reading, I kept wondering what Ringwood would say. “You mean, you didn’t investigate analogues first ?” was my surmise (but I imagine less politely put). Graff Jewelers (Bond Street, London) report that wealthy Russian shoppers prefer yellow diamonds (source: International Herald Tribune ). This is good …

High-pressure modifications of CaZn 2, SrZn 2, SrAl 2, and BaAl 2: Implications for Laves phase structural trends

High-pressure forms of intermetallic compounds with the composition CaZn 2, SrZn 2, SrAl 2, and BaAl 2 were synthesized from CeCu 2-type precursors (CaZn 2, SrZn 2, SrAl 2) and Ba 21Al 40 by multi-anvil techniques and investigated by X-ray powder diffraction (SrAl 2 and BaAl 2), X-ray single-crystal diffraction (CaZn 2), and electron microscopy (SrZn 2). Their structures correspond to that of Laves phases. Whereas the dialuminides crystallize in the cubic MgCu 2 (C15) structure, the dizincides adopt the hexagonal MgZn 2 (C14) structure. This trend is in agreement with the structural relationship displayed by sp bonded Laves phase systems at ambient conditions.

Structure−Property Relationships in the Crystals of the Smallest Amino Acid: An Incoherent Inelastic Neutron Scattering Study of the Glycine Polymorphs

Incoherent inelastic neutron scattering spectra for the three crystalline polymorphs (alpha- P2(1)/n, beta- P2(1), gamma- P3(1)) of glycine (C2H5NO2) at temperatures between 5 and 300 K (using the time-of-flight (ToF) spectrometer NEAT at HMI) and at pressures from ambient up to 1 GPa (using the ToF spectrometer IN6 at the ILL) were measured. Significant differences in the band positions and their relative intensities in the density of states (DoS) were observed for the three polymorphs, which can be related to the different intermolecular interactions. The mean-squared displacement, <u(2)>(T), dependence reveals a change in dynamic properties at about the same temperature (150 K) for all the three forms, which can be related to the reorientation of the NH3 group. Besides, a dynamic transition in beta-glycine at about 230-250 K on cooling was also observed, supporting previously obtained adiabatic calorimetry data. This behavior is similar to that already observed in amorphous solids, on approaching the glass transition temperatures, as well as in biological systems. It suggests the onset of degrees of freedom most likely related to transitions between slightly different conformational orientations. The DoS obtained as a function of pressure has confirmed the stability of the alpha-form with respect to pressure and also depicted a sign of the previously reported reversible beta-beta' glycine phase transition in between 0.6 and 0.8 GPa. Moreover, a remarkable kinetic effect in the pressure-induced phase transition in gamma-glycine was revealed. After the sample was kept at 0.8 GPa for an hour in the neutron beam, an irreversible transition into a high-pressure form (different from the beta'-form) occurred, although previously in X-Ray diffraction and Raman spectroscopy experiments a gamma- to delta-glycine phase transition was observed above 3.5 GPa only.

Novel high-pressure structures of MgCO3, CaCO3 and CO2 and their role in Earth's lower mantle

Most of the oxidized carbon in the Earth's lower mantle is believed to be stored in the high-pressure forms of MgCO3 and/or CaCO3 or possibly even CO2. Recently, through ab initio evolutionary simulations and high-pressure experiments, a complete picture of phase transformations of CaCO3 at mantle pressures was obtained. Here, using the same approach, we investigate the high-pressure structures of MgCO3. Two new structure types were predicted to be stable in the relevant pressure range: one at 82–138 GPa and the other above 138 GPa. Both phases contain rings of corner-sharing CO4-tetrahedra. These predictions were largely confirmed by the experiments presented here. A number of structurally very different, but energetically competitive metastable polymorphs were found and reveal complex high-pressure chemistry of MgCO3, in contrast to CaCO3. For CO2, from 19 GPa to at least 150 GPa, we find β-cristobalite structure to be stable. Differences between high-pressure tetrahedral carbonates and low-pressure silicates are discussed in terms of rigidity of the T–O–T angles (flexible when T=Si and stiff when T=C). We show that through most of the P–T conditions of the mantle, MgCO3 is the major host of oxidized carbon in the Earth. We discuss the possibility of CO2 release at the very bottom of the mantle, which could enhance partial melting of rocks and explain the geodynamical differences between the Earth and Venus.

Evidence for a high-density amorphous form in indomethacin from Raman scattering investigations

Pressure-induced transformation of $\ensuremath{\gamma}$-IMC [1-($p$-chlorobenzoyl)-5-methoxy-2-methylindole-3-acetic acid] is analyzed from Raman scattering investigations in the low-frequency range of $10--250\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and the high frequency region between 1550 and $1750\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, where $\mathrm{C}\mathrm{O}$ stretching vibrations are usually observed. At room temperature, by pressurization from atmospheric pressure up to $4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, $\ensuremath{\gamma}$-IMC undergoes a collapse transformation into a high-pressure crystalline form, induced by large rearrangement in the hydrogen-bonded network associated with molecular conformational changes. The Raman spectrum of the high-pressure crystal is similar to that of the $\ensuremath{\alpha}$ form, which is denser than the $\ensuremath{\gamma}$ form and metastable with respect to $\ensuremath{\gamma}$-IMC at atmospheric pressure. Upon further compression a solid-state amorphization is observed via the breakdown of hydrogen bonds. The Raman line shape of the high-pressure amorphous form is different from that of the vitreous state (or thermal glass obtained by quenching the liquid), suggesting the existence of a high-density amorphous state. By release of pressure, this high-density amorphous state transforms into the thermal glass. This transformation can be interpreted as a transformation between a high-density amorphous to a low-density amorphous state, which could be associated with a polyamorphic transformation.

Pressure-induced amorphization and decomposition ofFe[Co(CN)6]

The high-pressure behavior of the Prussian Blue analogue $\mathrm{Fe}[\mathrm{Co}{(\mathrm{C}\mathrm{N})}_{6}]$ has been explored by energy- and angle-dispersive x-ray diffraction, Fourier transform infrared, and Raman spectroscopy. This cyanide-bridged framework material becomes x-ray amorphous above $10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and the high-pressure form is retained upon pressure release. Samples recovered from around $17\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ exhibit a dark, metallic luster and their Raman spectra are characteristic of amorphous $\mathrm{C}{\mathrm{N}}_{x}$ films. These observations constitute strong evidence that $\mathrm{Fe}[\mathrm{Co}{(\mathrm{C}\mathrm{N})}_{6}]$ decomposes at high pressures.

Extraction and formulation of anthocyanin-concentrates from grape residues

The interest in natural food colorants shows an increasing tendency due to the strictening rules and consumers’ concerns over synthetic additives. One of natural pigments with high potential are anthocyanins, which are providing colours from yellow to blue in berries, flowers and vegetables. However the use of anthocyanins as food colorants is still limited because of their relatively low stability during processing, formulation and storage. In this work extraction of anthocyanins from grape marc was investigated with different concentrations of acetone (50 wt%, 70 wt% and 100 wt%) at 20 °C and at 60 °C temperatures. Pilot-scale extraction of grape marc was performed upon the results obtained from laboratory scale experiments. High-pressure concentrated powder form (CPF) technique was applied in order to formulate the obtained extracts in powderous form. Carrier materials, such as starch and silica were tested. Colorimetric properties (lightness, chroma and hue angle) of powderous products were measured and compared with synthetic colorants. Colour stability was monitored during storage at different conditions (light/dark and ambient temperature/refrigerator). The results were compared with the non-formulated extracts, which were stored at same conditions. Product formulation with CPF improved the colour stability of anthocyanin extracts, while for the non-formulated extracts degradation of colour rapidly occurred. The results show that formulated anthocyanin extracts have potential use as natural colorants. However, the major difference compared to synthetic colorants is still in the saturation of the colour.

Oxygen K-edge fine structures of water by x-ray Raman scattering spectroscopy under pressure conditions

Fine structure of the oxygen K edge was investigated for water at ambient pressure, 0.16, 0.21, 0.27, 0.47, and 0.60 GPa using x-ray Raman scattering spectroscopy (XRS). Similarity in near-edge structures at 0.16 and 0.60 GPa suggests little difference in the electronic state of oxygen in the low-pressure and high-pressure forms of water. Yet, we observed significant variation of preedge structure of the XRS spectra with compression. The intensity of the preedge peak at 535.7 eV has a minimal value at around 0.3 GPa, indicating that the number of hydrogen bonding increases first and then decreases as a function of pressure.

The High‐Pressure Form of Cadmium Vanadate, CdV2O6.

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Quantifying garnet-melt trace element partitioning using lattice-strain theory: assessment of statistically significant controls and a new predictive model

As a complement to our efforts to update and revise the thermodynamic basis for predicting garnet-melt trace element partitioning using lattice-strain theory (van Westrenen and Draper in Contrib Mineral Petrol, this issue), we have performed detailed statistical evaluations of possible correlations between intensive and extensive variables and experimentally determined garnet-melt partitioning values for trivalent cations (rare earth elements, Y, and Sc) entering the dodecahedral garnet X-site. We applied these evaluations to a database containing over 300 partition coefficient determinations, compiled both from literature values and from our own work designed in part to expand that database. Available data include partitioning measurements in ultramafic to basaltic to intermediate bulk compositions, and recent studies in Fe-rich systems relevant to extraterrestrial petrogenesis, at pressures sufficiently high such that a significant component of majorite, the high-pressure form of garnet, is present. Through the application of lattice-strain theory, we obtained best-fit values for the ideal ionic radius of the dodecahedral garnet X-site, r0(3+), its apparent Young’s modulus E(3+), and the strain-free partition coefficient D0(3+) for a fictive REE element J of ionic radius r0(3+). Resulting values of E, D0, and r0 were used in multiple linear regressions involving sixteen variables that reflect the possible influence of garnet composition and stoichiometry, melt composition and structure, major-element partitioning, pressure, and temperature. We find no statistically significant correlations between fitted r0 and E values and any combination of variables. However, a highly robust correlation between fitted D0 and garnet-melt Fe–Mg exchange and DMg is identified. The identification of more explicit melt-compositional influence is a first for this type of predictive modeling. We combine this statistically-derived expression for predicting D0 with the new expressions for predicting E and r0 outlined in the first of our pair of companion papers into an updated set of formulae that use easy-to-measure quantities (e.g. garnet composition, pressure, temperature) to predict variations in E, r0, and D0. These values are used in turn to calculate D values for those garnets. The updated model substantially improves upon a previous model (van Westrenen et al. in Contrib Mineral Petrol 142:219–234, 2001), and accounts well for trivalent cation partitioning in nominally anhydrous systems up to at least 15 GPa, including for eclogitic bulk compositions and for Fe-rich systems appropriate to magmagenesis on the Moon and Mars. The new model is slightly less successful in predicting partitioning with strongly majoritic garnets, although the mismatch is much less than with the original 2001 model. Although it also improves upon the 2001 model in predicting partitioning in hydrous systems, the mismatch between model and observation is still unacceptably large. The same statistical tools were applied in an attempt to predict tetravalent partitioning as well, because lattice-strain based techniques are not applicable to such partitioning. However, no statistically significant predictive relationships emerged from that effort. Our analyses show that future efforts should focus on filling the gap in partitioning data between ∼10 and 25 GPa to evaluate more closely the gradual transition of garnet to majorite, and on systematically expanding the hydrous partitioning database to allow extension of our model to water-bearing systems.

A high-pressure form of sulfuric acid monohydrate as determined by X-ray and neutron diffraction

Two high-pressure polymorphs of sulfuric acid monohydrate (oxonium hydrogensulfate) have been obtained at ambient temperature by crystallisation at high pressure from the liquid at 1.3 GPa (form III) and by direct compression of the ambient-pressure form I first to 1.26 GPa (form II) and then to 1.72 GPa (form III). The structure of form III was solved by single crystal X-ray diffraction and this structure was used as the basis for the refinement of hydrogen positions using high-pressure neutron powder diffraction data. Form III crystallises in the orthorhombic crystal system at 1.97 GPa, and features parallel chains of hydrogensulfate ions linked by oxonium ions to form a three-dimensional hydrogen-bonded network. On further compression to 3.05 GPa, the direction of maximum compressibility is found to be along the a-axis and is associated with the shortening of a hydrogen bond between a hydrogensulfate ion and an oxonium ion. The structure of form II remains elusive although at ambient temperature it is stable (or metastable) at pressures as low as 0.42 GPa, perhaps indicating that it could be recoverable to ambient-pressure at low temperature.

Electrical properties of pressure quenched silicon by thermal spraying

High velocity thermal spray deposition of polycrystalline silicon film onto single crystal substrates, yields metastable high pressure forms of silicon in nanocrystalline form within the deposit. The phases observed in the deposit include hexagonal diamond-Si, R-8, BC-8 and Si-IX. The peculiar attribute of this transformation is that it occurs only on <100> orientation silicon substrate. The silicon deposits containing the high pressure phases display a substantially higher electrical conductivity. The resistivity profile of the silicon deposit containing shock induced metastable silicon phases identified by X-ray diffraction patterns. The density of the pressure induced polymorphic silicon is higher at deposit/substrate interface. A modified two-layer model is presented to explain the resistivity of the deposit impacted by the pressure induced polymorphic silicon generated by the thermal spraying process. The pressure quenched silicon deposits on the p - silicon substrate, with or without metastable phases, display the barrier potential of about 0.72 eV. The measured hall mobility value of pressure quenched silicon deposits is in the range of polycrystalline silicon. The significance of this work lies in the fact that the versatility of thermal spray may enable applications of these high pressure forms of silicon.

In situ XAFS and XRD studies of pressure-induced local structural change in liquidAgI

In order to examine the pressure-induced structural change of liquid silver iodide (AgI),high-pressure and high-temperature in situ x-ray absorption fine structure (XAFS) and x-raydiffraction (XRD) studies have been carried out up to 1200 K and 6 GPa. The modificationsin the x-ray absorption near edge structure (XANES) spectra and x-ray structure factors,S(Q), with increasing pressure provide evidence for changes in the short-range order of liquidAgI. The I–Ag bond length in liquid AgI increases by compression up to 2 GPa, whichproposes that components with higher coordination than fourfold are introduced. The I–Agbond length decreases monotonically with compression above 2 GPa, indicating that thestructural change involving a coordination-number change is completed below 2 GPaand then a high-pressure form of liquid AgI is stabilized. Comparing the I–Agbond lengths of liquid AgI with those of crystalline phases, we conclude that thehigh-pressure form of liquid AgI has a rocksalt-like structure with large vacancies.

Ex situ XRD, TEM, IR, Raman and NMR spectroscopy of crystallization of lithium disilicate glass at high pressure

The structure of Li 2O · 2SiO 2 (LS 2) glass was investigated as a function of pressure and temperature up to 6 GPa and 750 °C, respectively, using XRD, TEM, IR, Raman and NMR spectroscopy. Glass densified at 6 GPa has an average Si–O–Si bond angle ∼7° lower than that found in glass processed at 4.5 GPa. At 4.5 GPa, lithium disilicate crystallizes from the glass, while at 6 GPa new high pressure form of lithium metasilicate crystallizes. This new phase, while having lithium metasilicate crystal symmetry, contains at least four different Si sites. NMR results for 6 GPa indicate the presence of Q 4 species with (Q 4)Si–O–Si(Q 4) bond angles of ∼157°. This is the first reported occurrence of Q 4 species with such large bond angles in alumina free alkali silicate glass. No five- or six-coordinated Si are found.