High-pressure Transformations Research Articles

High-Pressure Transformations and the Resonance Structure of Thiourea

High pressure increases intermolecular interactions in the crystal environment and affects the molecular dimensions of thiourea, CS(NH2)2, consistently with changing contributions of the thione and zwitterionic mesomers. The ambient-pressure phase V (space-group Pnma, Z = 4) at 0.34 GPa transforms to phase VI, of the same space-group symmetry, but with the larger unit-cell trifold (Z = 12). Another pressure-induced transition to phase VII was previously postulated for thiourea at 0.54 GPa; however, no associated structural transformations were identified. Our high-pressure single-crystal X-ray diffraction study reveals that following the transition at 0.34 GPa the lattice parameters and crystal structure strongly and monotonically change to about 0.65 GPa while the crystal symmetry is retained.

New clues from Earth’s most elusive impact crater: Evidence of reidite in Australasian tektites from Thailand

Abstract Australasian tektites are enigmatic drops of siliceous impact melt found in an ∼8000 × ∼13,000 km strewn field over Southeast Asia and Australia, including sites in both the Indian and Pacific oceans. These tektites formed only 790,000 yr ago from an impact crater estimated to be 40–100 km in diameter; yet remarkably, the young and presumably large structure remains undiscovered. Here we report new evidence of a rare high-pressure phase in Australasian tektites that further constrains the location of the source crater. The former presence of reidite, a high-pressure polymorph of zircon, was detected in granular zircon grains within Muong Nong–type tektites from Thailand. The zircon grains are surrounded by tektite glass and are composed of micrometer-sized neoblasts that contain inclusions of ZrO2. Each grain consists of neoblasts in three distinct crystallographic orientations as measured by electron backscatter diffraction, where all [001] directions are orthogonal and aligned with one <110> direction from the other two orientations. The systematic orientation relationships among zircon neoblasts are a hallmark of the high-pressure polymorphic transformation to reidite and subsequent reversion to zircon. The preserved microstructures and dissociation of zircon to ZrO2 and SiO2 require a pressure >30 GPa and a temperature >1673 °C, which represent the most extreme conditions thus far reported for Australasian Muong Nong–type tektites. The data presented here place further constraints on the distribution of high-pressure phases in Australasian tektites, including coesite and now reidite, to an area centered over Southeast Asia, which appears to be the most likely location of the source crater.

A new high-pressure form of Mg2SiO4 highlighting diffusionless phase transitions of olivine

High-pressure polymorphism of olivine (α-phase of Mg2SiO4) is of particular interest for geophysicists aiming to understand the structure and dynamics of the Earth’s interior because of olivine’s prominent abundance in the upper mantle. Therefore, natural and synthetic olivine polymorphs have been actively studied in the past half century. Here, we report a new high-pressure polymorph, the ε*-phase, which was discovered in a heavily shocked meteorite. It occurs as nanoscale lamellae and has a topotaxial relationship with the host ringwoodite (γ-phase of Mg2SiO4). Olivine in the host rock entrapped in a shock-induced melt vein initially transformed into polycrystalline ringwoodite through a nucleation and growth mechanism. The ringwoodite grains then coherently converted into the ε*-phase by shear transformation during subsequent pressure release. This intermediate metastable phase can be formed by all Mg2SiO4 polymorphs via a shear transformation mechanism. Here, we propose high-pressure transformations of olivine that are enhanced by diffusionless processes, not only in shocked meteorites but also in thick and cold lithosphere subducting into the deep Earth.

High-pressure transformation of MAPbI3: role of the noble-gas medium

High-pressure transformation of MAPbI₃: role of the noble-gas medium

High-pressure transformations of ortho-xylene probed by combined infrared and Raman spectroscopies

Here, we report high-pressure investigations of ortho-xylene (o-xylene) using combined infrared (IR) and Raman spectral measurements at room temperature and pressures up to 32.1 GPa, respectively. Liquid o-xylene visually crystallized at around 0.4 GPa. Upon compression, the spectra changed; including splitting, disappearing, broadening of modes, and the appearance of new modes. We found four more phase transitions in o-xylene to phases III, IV, V, and VI at around 1.4, 3.9, 12.3, and 16.8 GPa, respectively. The final product recovered from over 20 GPa was preserved at ambient conditions and the mass spectrometry experiment indicated phase transition are polymerization.

Discovery of the High-Pressure Phase of Ba3W2O9 and Determination of Its Crystal Structure.

A new polymorphism of Ba3W2O9 is discovered with the use of a high-pressure synthesis technique and its crystal structure is determined by single-crystal X-ray diffraction and transmission electron microscopy. The crystal structure was isostructural with that of Ba3Re2O9, having a hexagonal unit cell of R3̅m symmetry with a = 0.574060(10) nm and c = 2.08256(4) nm. The high-pressure (HP) phase is obtained from a transformation of an ambient-pressure (AP) phase of the compound, which has the Cs3Tl2Cl9-type structure. The most notable change in the transformation is the connection of WO6 octahedra. The HP phase has corner-sharing octahedra, which form a bilayer structure, while the AP phase has face-sharing octahedra of isolated [W2O9] dimers. This type of the structural phase transition is unreported although it is possibly that a sequence of high-pressure structural transformations occurs for similar chemical compositions. The HP phase has W ions in WO6 octahedra with an unusual off-center displacement; although the displacement is slightly relaxed compared with that of the AP phase. The off-center displacement suggests strong hybridization between the W 5d orbitals and O 2p orbitals.

Phase relations of Fe-Mg spinels including new high-pressure post-spinel phases and implications for natural samples

Phase relations of magnesioferrite-magnetite solid solutions (Mg,Fe 2+ )Fe 2 3+ O 4 were investigated at pressures of 9–23 GPa and temperatures of 1200–1600 °C. Our new results indicate that the assemblage Mg 2 Fe 2 O 5 + Fe 2 O 3 reconstitutes to a hp-MgFe 2 O 4 phase at 20 GPa and 1300–1500 °C. The stability field of hp-MgFe 2 O 4 begins at ~1300 °C and widens to higher temperature. At lower temperature (1200–1300 °C) Mg 2 Fe 2 O 5 + Fe 2 O 3 breaks down to the new phase assemblage Mg 3 Fe 4 O 9 + Fe 2 O 3 with its stability field expanding to higher pressures and temperatures at the expense of hp-MgFe 2 O 4 . The Mg 3 Fe 4 O 9 phase has the same crystal structure that recently reported for Fe 7 O 9 , and thus represents the Mg-end-member. From powder X-ray diffraction, we find that hp-MgFe 2 O 4 has a structure consistent with an orthorhombic unit cell belonging to the Pmcn space group (no. 62). However, it could have undergone a transformation from a different structure during decompression. Experiments conducted with a Mg 0.5 Fe 2+ 0.5 Fe 2 3+ O 4 composition demonstrate that the addition of Fe 2+ significantly changes the topology of the phase relations compared to the MgFe 2 O 4 end-member system. At 10–11 GPa and 1000–1600 °C, Mg 0.5 Fe 0.5 Fe 2 O 4 breaks down to the assemblage MgFeFe 2 O 5 + Fe 2 O 3 , with the phase boundary described by: P (GPa) = 2.0 × 10 − 3 × T (°C) + 8.2. No stability field for the constituent oxides [i.e., (Mg,Fe)O + Fe 2 O 3 ] exists, in contrast to that observed for the MgFe 2 O 4 end-member. The stability of the assemblage MgFe 2+ Fe 2 3+ O 5 + Fe 2 O 3 is limited at higher pressures and appears to pinch out to higher temperatures. At 15–16 GPa and temperatures up to 1350 °C, this assemblage reconstitutes to form a hp-Mg 0.5 Fe 0.5 Fe 2 O 4 phase. However, at higher temperatures a new assemblage of (Mg,Fe) 3 Fe 4 O 9 + Fe 2 O 3 appears. The occurrence of such compositions suggests that solid solution may be complete across the Mg 3 Fe 4 O 9 –Fe 7 O 9 binary. Our results further demonstrate that phase relations even in simple Fe-Mg oxides can become complex at high pressures and temperatures and that phases with various novel stoichiometries (i.e., Mg 3 Fe 4 O 9 ) may become stable. In addition, this study has implications for natural samples by helping to place constraints on the range in pressure and temperature at which a given sample formed. For instance, magnetite or magnesioferrite entrapped as inclusions in diamond could have either have crystallized directly, or formed from precursor phases at depths that exceed the stability of the spinel-structured phases. Evidence for such high-pressure transformations can potentially be found by investigating micro-textures.

Pressure-induced structural modifications of rare-earth hafnate pyrochlore

Complex oxides with the pyrochlore (A2B2O7) and defect-fluorite ((A,B)4O7) structure-types undergo structural transformations under high-pressure. Rare-earth hafnates (A2Hf2O7) form the pyrochlore structure for A = La–Tb and the defect-fluorite structure for A = Dy–Lu. High-pressure transformations in A2Hf2O7 pyrochlore (A = Sm, Eu, Gd) and defect-fluorite (A = Dy, Y, Yb) were investigated up to ~50 GPa and characterized by in situ Raman spectroscopy and synchrotron x-ray diffraction (XRD). Raman spectra at ambient pressure revealed that all compositions, including the defect-fluorites, have some pyrochlore-type short-range order. In situ high-pressure synchrotron XRD showed that all of the rare earth hafnates investigated undergo a pressure-induced phase transition to a cotunnite-like (orthorhombic) structure that begins between 18 and 25 GPa. The phase transition to the cotunnite-like structure is not complete at 50 GPa, and upon release of pressure, the hafnates transform to defect-fluorite with an amorphous component. For all compositions, in situ Raman spectroscopy showed that disordering occurs gradually with increasing pressure. Pyrochlore-structured hafnates retain their short-range order to a higher pressure (30 GPa vs. <10 GPa) than defect-fluorite-structured hafnates. Rare earth hafnates quenched from 50 GPa show Raman spectra consistent with weberite-type structures, as also reported for irradiated rare-earth stannates. The second-order Birch–Murnaghan equation of state fit gives a bulk modulus of ~250 GPa for hafnates with the pyrochlore structure, and ~400 GPa for hafnates with the defect-fluorite structure. Dy2Hf2O7 is intermediate in its response, with some pyrochlore-type ordering, based on Raman spectroscopy and the equation of state, with a bulk modulus of ~300 GPa. As predicted based on the similar ionic radius of Zr4+ and Hf4+, rare-earth hafnates show similar behavior to that reported for rare earth zirconates at high pressure.

Evidence of polymorphic transformations of Sn under high pressure**Project supported by the National Natural Science Foundation of China (Grant Nos. 11304294 and 11274281) and the Science Fund from the National Laboratory of Shock Wave and Detonation Physics of China (Grant Nos. 9140C670201140C67281 and 9140C670102150C67288).

The high-pressure polymorphs and structural transformation of Sn were experimentally investigated using angle-dispersive synchrotron x-ray diffraction up to 108.9 GPa. The results show that at least at 12.8 GPa β-Sn→bct structure transformation was completed and no two-phase coexistence was found. By using a long-wavelength x-ray, we resolved the diffraction peaks splitting and discovered the formation of a new distorted orthorhombic structure bco from the bct structure at 31.8 GPa. The variation of the lattice parameters and their ratios with pressure further validate the observation of the bco polymorph. The bcc structure appears at 40.9 GPa and coexists with the bco phase throughout a wide pressure range of 40.9 GPa–73.1 GPa. Above 73.1 GPa, only the bcc polymorph is observed. The systematically experimental investigation confirms the phase transition sequence of Sn as β-Sn→bct→bco→bco+bcc→bcc upon compression to 108.9 GPa at room temperature.

High-pressure and high-temperature transformation of Pb(II)-natrolite to Pb(II)-lawsonite.

We report on high-pressure and high-temperature chemical transformations of Pb(2+)-exchanged natrolite (Pb-NAT, Pb8Al16Si24O80·16H2O) using a combination of in situ synchrotron X-ray powder diffraction and ex situ HAADF-STEM real space imaging. Three high-pressure polymorphs of natrolites (Pb-NAT-I, II, III) are observed via step-wise pressure-induced hydrations (PIH) up to 4.5 GPa, during which the number of H2O molecules located inside the natrolite channel increases from 16 to 40 H2O per unit-cell. At 4.5 GPa after heating the high-pressure Pb-NAT-III phase at 200 °C a reconstructive phase transits into a lawsonite phase (Pb-LAW, Pb4Al8Si8O28(OH)8·4H2O) with an orthorhombic space group Pbnm and a = 5.8216(9), b = 9.114(1) and c = 13.320(1) Å is observed. The structure of the recovered Pb-LAW phase was characterized using Rietveld refinement of the in situ synchrotron X-ray powder diffraction data and HAADF-STEM real space imaging. In the recovered Pb-LAW phase the Pb(2+) content is close to 42 wt% and as bond valence approximations reveal the Pb(2+) cations are more tightly coordinated to the framework oxygen atoms than originally in the natrolite phase.

High repetition rate femtosecond laser irradiation of fused silica studied by Raman spectroscopy

In this article, we have focused our attention on the femtosecond laser irradiation of fused silica to study the mechanisms of micro-cavities and nanofibers formation. The nanofibers formed have small diameters and are extremely long, with a typical aspect ratio of 1000. Structural rearrangements and density variation have been studied by micro-Raman spectroscopy as a function of the irradiation parameters. 3D spatial characterizations of structural reorganizations and molecular oxygen production have given insights on high-temperature and high-pressure silica transformations by high repetition rate femtosecond laser irradiation.

Jadeite in the Gran Paradiso massif: the high-pressure metamorphic evolution of upper continental crust

The formation and eventual preservation of high-pressure mineral assemblages in subducted continental crust are a long-standing issue with kinetic aspects and important geophysical implications in terms of buoyancy. The issue is especially acute in the internal crystalline massifs of the Western Alps, in which the dominant rock-type, metagranite, hardly bears evidence for high-pressure transformation whereas subordinate rock types clearly point to high or very high pressure conditions. The discovery of well-preserved jadeite-bearing rocks in orthogneiss of the western Gran Paradiso massif, reported here, revives the questioning and is the opportunity to describe uncommon mineral assemblages involving nepheline, preiswerkite, as well as a new trioctahedral mica.

Serpentines, talc, chlorites, and their high-pressure phase transitions: a Raman spectroscopic study

Raman spectra of magnesian phyllosilicates belonging to the serpentine, talc, and chlorite groups have been obtained at ambient conditions, and at high pressures and up to 200 °C in order to study high-pressure transformations in the 10 GPa range. The complex and distinct Raman spectra of these minerals allow straightforward identification, which may otherwise be difficult from optical microscopy. High-pressure measurements are in good agreement with DFT calculations for talc and lizardite. Pressure-induced displacive modifications are identified in lizardite and antigorite serpentines, and in chlorite at ~4, 7 and 8 GPa, respectively, while talc shows no transition up to ~11 GPa. At high temperature, the high-pressure distortions of serpentines shift to higher pressures. Given the stability limits of these minerals, and the natural range of P–T conditions, none of the high-pressure distortions observed at high pressure are likely to occur at depth in the Earth.

Franciscan geologic history constrained by tectonic/olistostromal high-grade metamafic blocks in the iconic California Mesozoic-Cenozoic accretionary complex

![Figure][1] Subduction generated a NW-trending Andean arc along the Californian margin beginning at ~175 Ma. Coeval high-pressure (HP) transformation of oceanic crust in an east-dipping, subduction zone probably accompanied plate convergence, but recovered Franciscan eclogite and garnet blueschist blocks chiefly possess younger recrystallization ages of ~165–150 Ma. These Jurassic HP metamafic rocks were sequestered in a low-temperature environment well into Cretaceous time. Fragments of actinolitic rinds partially surround many such high-grade blocks. Only slightly younger than the HP metamorphism, these rinds reflect metasomatic exchange between metabasalt and serpentinized harzburgite along the dynamic oceanic crust-mantle hanging wall during storage of the mafic rocks at moderate depth. High-grade tectonic blocks later were brought toward the surface in circulating, low-density, sheared mud-matrix melange and/or in buoyant serpentinite bodies. Most exotic HP metamafic blocks occur in melanges of the Franciscan Central Belt, reflecting tectonic insertion within the subduction zone—not near-surface additions to the clastic section. However, rare, high-grade clasts in feebly recrystallized Franciscan conglomerates suggest erosion and sedimentary deposition for some HP blocks. The addition of dense metabasaltic olistoliths to the mid- and Upper Cretaceous section requires that these HP material were carried surfaceward first as tectonic fragments, perhaps immersed in low-density serpentinite or mud-matrix melange, then eroded and transported into the trench. HP rocks are conspicuously lacking in coeval Great Valley strata. Whatever the origin of particular high-grade rocks, widespread post-depositional shearing has largely obliterated their original natures, but all dense metamafic blocks of Jurassic recrystallization age must have been supplied to the Cretaceous Franciscan accretionary complex by entrainment in a low-density, circulating muddy matrix or serpentinite body. The vast majority of exotic HP blocks resides in Central Belt melanges, and appears to be tectonic rather than olistostromal in origin. [1]: pending:yes

High-pressure transformation in the cobalt spinel ferrites

We report high pressure angle-dispersive x-ray diffraction measurements on CoxFe3−xO4 (x=1, 1.5, 1.75) spinels at room temperature up to 34GPa. The three samples show a similar structural phase transformation from the cubic spinel structure to an analogous post-spinel phase at around 20GPa. Spinel and post-spinel phases coexist in a wide pressure range (~20–25GPa) and the transformation is irreversible. The equation of state of the three cubic spinel ferrites was determined and our results agree with the data obtained in related oxide spinels showing the role of the pressure-transmitting medium for the accurate determination of the equation of state.Measurements releasing pressure revealed that the post-spinel phase is stable down to 4GPa when it decomposes yielding a new phase with poor crystallinity. Later compression does not recover either the spinel or the post-spinel phases. This phase transformation induced by pressure explains the irreversible lost of the ferrimagnetic behavior reported in these spinels.

High-pressure structural transformations of PbCrO4 up to 51.2 GPa: An angle-dispersive synchrotron X-ray diffraction study

We report on high-pressure X-ray diffraction measurements up to 51.2GPa in PbCrO4 at room temperature. Three high-pressure phases with structures different than the ambient-pressure monazite-type (P21/n) are reported. One phase transition was found at 3.8GPa to an isomorphic structure to monazite. A second transition occurs at 11.1GPa. After this transition, the coexistence of tetragonal (I41/a) and monoclinic (P21) structures is detected up to 21.1GPa. Beyond this pressure and up to 51.2GPa, only the high-pressure monoclinic phase is observed. Upon decompression all structural changes are reversible. Finally, the axial compressibilities for the different phases have been determined as well as the equations of state.

Pressure and Laser-Induced Reactivity in Crystalline s-Triazine

The high-pressure transformation of s-triazine to an extended amorphous carbon incorporating a large amount of nitrogen has been studied by Fourier transform infrared spectroscopy. The reaction, occurring at ambient temperature around 8 GPa, is found to be extremely sensitive to temperature, as the reaction onset is lowered to 4 GPa at 530 K. Also, laser irradiation allows a very selective trigger of the reactivity below the ambient temperature threshold likely due to the population of ππ* states. Independently of temperature and irradiation conditions, two different products are obtained once the reaction is activated at 3.5–4.0 GPa or above 8 GPa. The low pressure product is likely composed by conjugated domains forming in the monoclinic ab planes, whereas with rising pressure interactions between the planes destroy the sp2 clusters increasing the concentration of N bearing terminating groups and severely affecting the mechanical properties of the extended material.

Onion-like carbon in impact diamonds from the Popigai astrobleme

Phase composition and nanostructure of layered and irregular massive impact diamond grains from the Popigai astrobleme have been investigated by Raman spectroscopy and high-resolution electron microscopy and the conditions of phase transformations are discussed. Several coexisting carbon phases forming tight aggregates have been found, including cubic and hexagonal diamond polymorphs, graphite, amorphous carbon, fullerene-like/onion-like carbon. The latter is described within impact diamonds for the first time. It is proposed that the formation of onion-like carbon, in both layered and massive impact diamond grains, is connected with high-pressure graphite transformation or post-pressure stress cooling which causes the partial back transformation of diamond nanocrystallites to sp2 carbon. However, a possible relict origin of the fullerene-like carbon from the impacted initial sedimentary rocks with fullerenes and fullerene-like substances, like shungite or coal, can not be excluded. The difference between micro- and nanostructures of layered and irregular massive impact grains is presented.

High-Pressure Transformations in Natural Product Synthesis

This review summarizes the application of high-pressure chemistry in natural product synthesis. By highlighting reactions that could not be promoted by conventional means but that proceed readily under high-pressure conditions, we intend to disclose the benefits of high-pressure-mediated reactions for organic transformations. The individual syntheses are classified by the reaction type of the key step and are discussed briefly. 1 Introduction 2 Cycloaddition Reactions 2.1 Diels–Alder Reactions 2.2 Inverse-Electron-Demand Diels–Alder Reactions 2.3 Hetero-Diels–Alder Reactions 3 Miscellaneous Reactions 4 Conclusion

High-Pressure-Induced Polymorphic Transformation of Maleic Hydrazide

Maleic hydrazide, a unique example of polymorphic structures, was analyzed at pressure up to 5 GPa using in situ high-pressure Raman scattering and synchrotron X-ray diffraction techniques. Changes in the Raman spectra at 2 GPa indicate that a pressure-induced phase transition is occurring. The transition was further analyzed with angle dispersive X-ray diffraction, which demonstrated that maleic hydrazide underwent a polymorphic transformation from the form III to the form II. Moreover, the observed transformation was partially reversible when the system was brought back to ambient pressure. This work suggests that the high-pressure polymorphic transformation is caused by changes in the hydrogen-bonded ribbons which lead to supramolecular rearrangements in the crystal structure.